NATIONAL BUREAU OF STANDARDS REPORT

10 610

Progress Report

on PETROGRAPHIC STUDY OF THE REFRACTORY

PERFORMANCE OF HIGH-FUSING DENTAL ALLOY

INVESTMENTS. PART I: HIGH-FUSING PHOSPHATEBONDED INVESTMENTS, SYNCHRO-X INVESTMENTS

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

311.05-11-31 10189 September 14, 1971 10 610

Progress Report

on PETROGRAPHIC STUDY OF THE REFRACTORY

PERFORMANCE OF HIGH-FUSING DENTAL ALLOY

INVESTMENTS. PART I: HIGH-FUSING PHOSPHATEBONDED INVESTMENTS, SYNCHRO-X INVESTMENTS

by

C. P. Mabie*

Research Associate, American Dental Association

Research Unit at the National Bureau of Standards,

Washington, D. C. 20234.

This investigation is part of the dental research

program conducted by the National Bureau of Standards

in cooperation with the American Dental Association;

the United States Army Medical Research and Development Command; the Dental Sciences Division of the

School of Aerospace Medicine, USAF; the National

Institute of Dental Research and the Veterans Administration.

NATIONAL BUREAU OF STANt

for use within the Government. Befc and review. For this reason, the pu whole or in part, is not authorized Bureau of Standards, Washington, D

the Report has been specifically pref

IMPORTANT NOTICE Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

ccounting documents intended

lected to additional evaluation

ing of this Report, either in

fice of the Director, National

e Government agency for which

s for its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Petrographic Study of the Refractory

Performance of High-Fusing

Dental Alloy Investments

PART I. — High-Fired Phosphate-Bonded Investments

Detailed X-ray and microscopic petrographic studies of

high-fusing alloy investments from the viewpoint of their

bonding mechanisms and their reactions with the casted alloy

as they pertain to determining refractory performance have

been carried out. It is thought that this knowledge will

contribute to the improvement of investment materials.

Both phosphate and silica-bonded investments are used in

the casting of high-fusing dental alloys. This report gives

results of a study of a coarse-grained high-fired phosphatebonded investment, Synchro-X*. This investment was selected

for study primarily because it has been used extensively in

high-temperature burnouts and fires where the refractory performance

of phosphate-bonded investments are tested most severely.

Electron-probe and X-ray diffraction studies have been made

by Allan and Asgar1 to determine the reactions of Co-Cr-base

alloy with a fine-grained phosphate-bonded investment containing

a colloidal silica binder, Cermigold. Also in their study,

Allan and Asgar investigated, primarily by X-ray diffraction.

* The National Bureau of Standards does not approve, commend, or

endorse any proprietary product or material as a class or group,

and results reported by the Bureau shall not be used in advertising

or sales promotion, or to indicate either explicitly or implicitly

endorsement of the product or material by the National Bureau of

Standards

.
.

H

'

-
-2ainvestment-bonding reactions at various temperatures. However,

more detailed microscopic work on the phosphate-bonded investments

appeared merited. It also seemed that better X-ray identification of

phases in the fired investment could be obtained by making more

effort in the preparation of concentrates. These were prepared

by magnetic fractionation with the Franz Isodynamic Separator,

selective sizing with sieves, and heavy liquid separation using

bromoform-containing solutions. Small samples of powdered

concentrates were mounted as glue-cement spindles in a DebyeScherrer camera and subjected to prolonged, 10 to 15 hours, exposures to nickel-filtered copper X-radiation.

The manufacturer states that during burnout Synchro-X

investment should be fired to 920 °C over a period of 1.5 to 2

hours. This investment has been recently replaced by a finergrained phosphate investment, Synchro-X-" L" , that is to be fired

to 705-732 °C. It is said to produce a smoother cast than the

older Synchro-X and is very similar in size constitution to the

Cermigold, but is not used with a colloidal silica binder.

In practice, however, dental laboratories commonly tend to

fire investment-mold casings used for high-fusing alloys to the

highest permissible furnace temperature. In this work the temperature used was 1000 °C ± 50° —attaining this temperature with a fully loaded kiln in approximately 3.5 to 4.0 hours and with

1 to 1.5 hour soaks prior to casting. One reason why these

high temperatures are used is that gas-fired kilns commonly are
-2bheavily loaded. This heavy loading puts a severe burden on these

small kilns leading to large temperature gradients both inside

the kiln and inside the individual mold casings. A hightemperature setting will assure that both the outside and the

inside of all molds are adequately heated in the shortest time

possible and will increase the allowable time for casting.

Secondly, a high-temperature set will assure that the furnace

temperature will not drop below acceptable limits during

rapid removal of mold casings.
-3-

These dental laboratory procedures make imperative the

study of the high-temperature reactions of investments. To

meet this need petrographic characterizations of the investment

both before and after burnout and fire, and both at and near the

investment-casting interface were made.

Characterization of the Unfired Investment : One part of

Synchro-X liquid commonly is mixed with approximately four parts

of investment powder by weight. This ratio varies depending upon

whether the model or the outer investment is being prepared,

and varies, in making the outer investment, with the diameter

of the casing c

. Unlike the liquid used in silicate investments,

Synchro-X liquid is almost all water and adds very little solid

to the investment. The liquid is a dilute alkaline solution of

sodium, copper, chloride, sulphate, and possibly other ions.

When liquid is mixed with investment powder, the mixture readily

air sets.

Petrographic and chemical analyses reveal that the investment

powder was predominantly quartz and cristobalite (75 to 85%) with

subordinate amounts of monoammonium phosphate (10%) and calcined

magnesite, and trace amounts of heavy mineral constituents normally found in sand. Quartz and cristobalite occur in approximately equal amounts, with quartz being slightly more abundant.

No tridymite was detected (Table II) . Both have approximately

the same size distribution, predominantly ranging in size from

.particles

100 to 10 qm, but some^ are finer. Most of the quartz grains are
-4-

tabular to prolate in shape and those of cristobalite are blocky

to spherical (Figure 1) . Minutely disseminated impurities,

quartz blebs and gas bubbles caused the cristobalite to appear

cloudy (Figure 2) . A few grains are complex intergrowths of

quartz and cristobalite.

The cristobalite is complexly twinned and is very finely

submicrocrystalline to cryptocrystalline. Usually the cristobalite is faintly birefringent but in places appears isotropic,

possibly because of overlap twinning resulting from inversion

obi

./

cristobalite 3

. from high-temperature-stable7

. Only an average refractive index can

be measured because of the fine crystallinity and minutely spaced

twinning. This refractive index is 1.484 (±0.001) , typical of

some synthetic low-temperature-stable cristobalite4

. The monoammonium phosphate constituent (NH 4 H 2 P0 4 ) is

recognised immediately by its strong birefringence, n - n =

o,o V^rV o e

- ^-gatSES^ refractive indices, n

Q = 1.5246 and n

g = 1.47 92, and

uniaxial negative polarization5

. It occurs as subspherical

particles commonly ranging from 70 to 20 pm in diameter, with

some finer, but infrequently up to 200 pm.

Periclase (MgO) is the predominant phase in the crushed

magnesite-clinker constituent. Subordinate amounts of monticellite (CaMgSi0 4 ) envelop the periclase in places and help bond

it into grape-like clusters (Figure 3) . The high and low

refractive indices of the monticellite are n = 1.636 ± 0.001 X

and n = 1.652 ± 0.001 with n - n = 0.016 ± 0.002. These

z Z X

are essentially the same as values reported for pure iron— free
-5-

monticellite 5

. X-ray data indicate that beside periclase

and monticellite very subordinate amounts of forsterite, Mg 2 Si0 4 . are present (Table II) . It was difficult positively to detect

this phase microscopically. Individual periclase crystals commonly occur as spheres ranging from 10 to 20 pm in diameter

and frequently have diameters up to 35 and down to 10 pm. The

refractive index of periclase is most commonly close to 1.75.

Light yellow periclase ranges slightly lower, and the deep

yellow and brown periclase with higher iron contents ranges up

to 1.80. The periclase contains finely disseminated sub-

(FeO!- -MgOn -Fe 2 0 3 ) microcrystalline magnesioferr ite' which is an invariable constituent

of iron-rich deadburned magnesite.

Spectrographic analyses are shown in Table I. Almost all

of the silicon occurs as silica in quartz and cristobalite with

only a very minor amount occurring as monticellite. The

magnesium is predominantly as periclase and to a minor extent

as monticellite, which contains almost all of the calcium.

The phosphorus occurs solely as monoammonium phosphate. Aluminum,

iron and manganese probably are located mostly in the calcined

magnesite. Chromite (FeCr 2 0 4 ) is sometimes added in the calcination

of magnesite and is the probable source of most of the chromium1"

. Boron, copper, potassium, lithium and sodium probably originate

mostly in the liquid. Titanium occurs at least partly as the

very rare rutile and ilmenite contaminants associated with the

silica, but may be present to a large extent in the calcined
- 6 -

magnesite. The zinc is probably mixed into the investment as

a very fine-grained calcined zinc oxide.

Characterization of the Burned Out and Fired Investment ; When

investment powder is mixed with liquid, the monoammonium

phosphate is readily dissolved. Upon setting a gel-like appearing bo

envelops many of the particles (Figure 4). Heating at 100 °C

causes ammonia to be evolved and ammonium and other alkali

condensed phosphates to be left behind to give green strength.

The very minor amounts of zinc oxide and free lime present in

the calcined magnesite and possibly in the fine-grained periclasa

may contribute to the setting of the phosphate bond.

A tightly-bonded body results when Synchro-X investment

is fired at 1000 °C ± 50° (Figures 5 to 7) . The cold strength

of this burned out phosphate-bonded investment is greater than

that of silica-bonded investments. This greater strength is

caused primarily by a higher degree of sintering due to fusionpromoted silica mobilization.

Examinations of polished and thin sections show that

quartz and cristobalite grains exhibit irregular "sticky"

margins (Figures 7 and 8) . Quartz grains, commonly, but not

everywhere, have isotropic-appearing rims with measurable refractive indices between 1.482 and 1.486 (Figure 8) . Matrix between

quartz and cristobalite grains has an isotropic-appearing aspect

with refractive indices ranging between 1.482 and 1.490 (Figure

9) . Transmitted light reveals that minute quartz crystals may
-7-

be incorporated in this matrix. Also, X-ray patterns and

microscopically determined crystal shapes and indices indicate

tht tridymite may be present as part of the matrix (Table III,

XRD-12, 24).

In some interstitial areas considerable "liquid bridging"

occurs between the silica particles (Figure 7) . Sinter bonds

in these mobilized interstitial areas, not highly infused with

magnesite clinker material, appear to consist of recrystallized

silica, quartz and possibly tridymite interwoven with a vitreous

constituent that appears in places to contain dendrites. This

dendritic character is very noticeable in the interparticle bond

constituents mobilized in the casting process.

The intimate interstitial dispersion of phosphate through

the investment promotes silica fusion. This is expected since

a low temperature eutectic in the binary system P 3 0 5 -Si02

occurs well below 1085 °C and probably below 1000

7

. Salt

constituents present in the investment liquid (e.g., Na, Cl,

Li, CuO, S0 3 ) are intimately co-precipitated with the phosphate

and, along with fine-grained zinc oxide and possibly solulalized

lime, probably help greatly to lower fusion temperatures in

interstitially occurring bond micro-environments.

Microscopic examination of high-fired investment in both

polished and thin sections reveals that micro-environments around

particles of magnesite clinker are highly fused (Figures 10 to

17) . Fusion in these regions appears much greater than elsewhere.
- 8 -

Monticellite from the magnesite clinker is partly to completely

converted to a sub microcrystalline, moderately birefringent

phase with irregular elongated crystals (Figures 15 to 17)

.

Relict monticellite in the fired investment has refractive

indices slightly higher, commonly n = 1.659 and n = 1.637 ±

Z X

0.002, than in the original magnesite clinker. Refractive

indices of the submicrocrystalline reaction product appear to

be variable. Most of the low indices are between 1.622 and

1.626 and the high between 1.634 and 1.636.

X-ray patterns of concentrates particularly rich in the

submicrocrystalline reaction product, XRD-10, 11, 14, 15 and 16,

yield lines which are those of forsterite and others which were

unidentified (Table . However, the optical properties

and general occurrence of most of the reacted material that has

degraded in place from monticellite resemble those of 5CaO*

2MgO*6SiO s for which no pattern has been reported; n = 1.621,

ri = 1.627, n^ = 1.635, 2V = approximately 80°+.

According to Ferguson and Merwin 8

, "Mix crystals of

wollastonite (CaSi0 3 ) and 5CaO* 2MgO* 6Si0 3 and also 2CaO*MgO* SiO ? (akermanite) have optical properties which are intermediate so far

as they have been determined" . X-ray patterns indicate that

additional forsterite is formed wfyen MgO reacts with

the surrounding silica (XRD-10, 14, 15 and 16 of Table IV)

.

Periclase (MgO) relicts in partly fused magnesite clinker

have deep caries and embayments consisting of the rimming submi croqry stall ine calcium-magnesium-silicate reaction product
-9-

(Figures 10, 11, 15 to 17) . Magnesite-clinker concentrates from

both fired and unfired investment were found to be highly magnetic

indicating that some magnetite (Fe0*Fe 2 0 3 ) and/or magnesioferrite

(Fe0 1 _ n 'MgOj. *Fe 2 0 3 ) is present. Reaction zones around corroded

periclase, as indicated by a reddish nearly opaque appearance in

the thin section of Figure 17, commonly have high concentrations

of what appears to be finely-divided magnesioferrite associated

with much lesser amounts of probable hematite (Fe 2 0 3 ) . Also spinel

(Mg0*Al 2 0 3 ) might be present in these zones. Close examination

of the polished section photomicrographs (Figures 10 and 11) shows

minute bright and apparently anisotropic corroded crystals proximal

to relict periclase which may be hematite intimately mixed with

magnesioferrite . Where periclase corrosion is highly progressed,

caried and embayed minute magnesioferr ite and hematite relicts

appear to remain (Figure 12)

.

Liquefaction streamers from magnesite-clinker particles

penetrate deeply into the surrounding investment. Submicroscopic

,

finely-dispersed iron oxide gives these "streamers" a yellowish

appearance (Figure 14) . Lath-shaped crystals (10 to 20 x 2 to

5 qm) of unknown identity are present in small amounts in these

mobilized interstitial places. They have a high index (1.608 1 0.004)

and low index (1.566 ± 0.004/ and are length slow with a slightly

inclined extinction. The concentration of reddish ferruginous

constituents at the periphery of periclase corrosion (Figure 17)

and in deep liquefied embayments in periclase (Figure 14) indicates

that magnesioferrite aided in fluxing the periclase. " Liquefied
- 10 -

streamers" radiating from magnesite clinker connect with other

small "liquefied micro environments" and promote the transport of

magnesium, calcium, and iron oxides (Figures 10 to 14)

.

That the oxide components in the calcined magnesite, particularly

those of iron, promote liquefaction and sintering can be

understood when it is realized that the binary system Fe 2 0 3 -

P 2 0 5 has eutectics at 968 and 964 °C 9

. The even lower eutectic

which occurs in the system Fe0*Fe 2 0 3 -P 2 0 5 at 940 °C

10 makes it

understandable how magnesioferrite or magnetite may promote

fluxing. Reduced oxygen pressure induced by the manner in which

the gas kiln is regulated, residual wax atmosphere, or the addition

of graphite, as is done in one phosphate investment, may promote

magnetite and wustite (FeO) formation, and therefore, possibly

•t more extended silica and phosphate mobilization. Also, any

excess lime that may be present in the calcined magnesite can

initiate clinker fusion because in the system Ca0-P 2 0 5 a low

melting eutectic (500°C) occurs with very small Ca0-P 2 0 E ratios 11

. Thus, bonding of the Synchro-X investment occurs in the

following stages:

1. Monoammonium phosphate is dissolved and on setting

is coprecipitated with alkali ions including sodium, solubilized

zinc oxide, any free and soluble lime that mey be present in

the deadburned magnesite, possibly a very minute portion of the

MgO from the same source and chloride, sulfate t and other ions

present in the liquid mix.
- 11 -

2. Ammonia is liberated under low heat of a drying oven

a

or sunlamp, and/condensed-phosphate bond is formed.

3. Phosphate and silica fusion induced during burnout

and fire by the presence of sodium, chloride, lithium, sulfate

and other ions, form5 sinter bonds containing quartz, tridymite

and a glassy-appearing phosphate-silica constituent.

4. Periclase is fluxed by magnesioferrite in conjunction,

with fluxing by phosphate and silica, forming , along with MgO

removed from degraded monticellite , additional forsterite,

and from aluminum oxide included in the calcined magnesite,

possible magnesium aluminum spinels (Mg!

_

x Fe x Al 2 0 4 )

.

5. Part of the magnesioferrite is back oxidized to what

is probably hematite during the processes of step 4.

6. Further melt action occurs, leading to corrosion of

periclase, silica, and the melt diffusion of ferrous and ferric

oxide into the investment matrix and added promotion by dissolved iron oxide of even more extended fusion.

Characterization of Investment Surface Topography ;

The surface topography of investment-free partial-deqture

cases that have not been sandblasted is a network of minufe

hills and valleys. Microscopic examination of the investjnentcastir^g surface of investment fragments that have been knocked

free from poorly adhering alloy reveals that they also exhibit

a network of hills and valleys. These key into those of the

casting and are determined by the configuration of the particle
- 12 -

clumps along the surface of the casting-investment interface.

Effects of firing on investment-surface topography were

determined by microscopic examination of the surfaces of small

blocks of investment that had been fired to three different

temperatures. Fourteen investment bricks (7 x 5 x 1-1/2 cm) were

formed from Synchro-X standard refractory mix, poured into molds

floored by one centimeter thick refractory brick and walled by

two centimeter high walls of boxing wax. The liquid-powder mix

was vibrated in molds to release bubbles and to obtain flat

upper surfaces. After the poured investment was air dried overnight, the boxing wax was peeled off. Then the blocks were

heated at 100 °C for two hours in a drying oven. Upon removal

from the oven, the blocks smelled heavily of liberated ammonia.

Four blocks were sawed in half and these halves were

heated for one hour from room temperature to 560 °C ± 10°.

Four of the halves from different bricks were then transferred

into a silicon carbide tube furnace set at 560 °C and fired for one

hour until a temperature of 940 °C ± 10 °C was

attained. Another set of ten entire cast bricks were fired in a

gas kiln in a dental laboratory according to that laboratory's

procedure for Durallium alloy. This was a two-hour fire to a

kiln temperature of 1050 °C ± 50°.

Microscopic examination was made of the flat upper block

surfaces with a 50X-dry Leitz Ultropak objective. The settings

of the incident light cone and obstructing pie diaphragm were

kept the same for comparative examinations.
-13-

Examinations revealed that clumping of particle aggregates

increased as the severity of the firing conditions increased.

This may be seen by close comparison of the microphotographs

(Figure 18) . Dark areas in the photographs are valleys, and

the bright cloud-like areas are ridges and hills. In the

1050 °C fire, knobby areas tended to form about particles of

calcined magnesite. Therefore, fusion during the high fire led

to an overall increase in the size of topographic irregularities.

It is to be expected that burnout-firing history will have

a pronounced effect on the composition of the incipient melt

developed in the investment. Incipient-melt composition and

temperature will affect both the surface tension of the melt and

its interparticulate penetration. Both of these factors determine particulate clgmping or configuration. Therefore, it is

expected that the more extensive the sinter liquefaction, induced

by the combined operations of mold preheating and casting, the

more severe will be investment- topographic irregularities qt the

casting interface and the greater the casting roughness.

Characterization of Casting Effects : The casting of high-fusing

alloys into investments has two main effects, corrosive and

strictly thermal. Strictly thermal effects result solely

from the heat emanated from the casting and may occur partly

after alloy solidification. Corrosive effects are caused by

the combined effects of heat and oxidized vapors emitted by

the casted alloy.

i
-14-

Heat dissipation by the casted melt commonly was observed

in this study to heavily mobilize silica constituents 0.5 to 2

millimeters back of the casting-investment interface (Figures

19 to 23) . This is expected since Synchro-X investment is used

in the casting of Durallium alloy and this alloy does not form

a very fluid bead much below 1550 °C. It was experimentally

a

determined that/temperature of 1550 °C is sufficient to cause very

extensive silica mobilization in Synchro-X. Fusion resulting from

strictly thermal casting effects yields a sinter similar in

constitution to those bonds formed in the high fire, i.e.,

bonds containing predominantly recrystallized silica and

vitreous constituents —-compare Figures 9 and 22.

Simple thermal fusion effects back from the casting

interface do not appear extensive enough to reduce gas permeabi lity

significantly. However, different parts of the casted melt may

give off differing amounts of heat. The variability in the

amounts of this heat may make the investment susceptible to

anisotropic deformation immediately after alloy impact. It is

possible that this phenomenon could be a cause of some poor

denture fits.

Corrosive Effects ; Corrosive effects result from diffusion of

constituents from the molten alloy at high temperatures into

the investment. Commonly, these effects are evident in SynchroX investments 10 to 60 pm back of the interface and involve

extended liquefaction (Figures 21 through 23)

.

The results
- 15 -

of electron-probe examinations of these interfaces agree closely

with those reported for phosphate investments by Asgar1

. It

was found that:

lo Chromium had diffused the most into the investment.

2. Cobalt occurred in relatively small amounts in the

investment, out <f proportion to its major constitutional

status in the alloy, and was not nearly as abundant or

deeply diffused as the chromium.

3. Manganese had diffused into the investment as deep as

or deeper than the chromium, up to 80 gm, and was

nearly completely vaporized from alloy immediately

adjacent to the investment.

4. Iron appears to have diffused into the investment somewhat, apparently about as deep and as much as the cobalt.

5. No nickel, molybdenum, tungsten, copper, titanium or

investment

tantalum was detected in they interface

.

Since chromium and manganese commonly occur in phosphate

investments, it was thought that confirmation of the above

f results with an investment containing neither of these elements

was desirable (Table I) . Further, it was thought that prolonged

contact with a molten alloy bead under a low oxygen pressure

would make electronprobe interf acial-diffusion results somewhat

more definitive. For this purpose, a 7 x 5 x 1-1/2 cm block of

Howmet silica-bonded Rapid Set investment was prepared according

to the procedure used in preparing Synchro-X blocks, but with
-16-

the mixing procedure standard for silica-bonded investment.

To the set investment block a 0.25 cm thick Howmet protective

coat (" Pro-G" ) was applied, which consisted mostly of free

silica but contained appreciable amounts of zircon. After

drying and setting, the coated investment block was subjected to

a 1.5 hour gas kiln fire to 1177 °C. A Durallium alloy shot was

placed on this block and inserted into a silicon-carbide

mullite-tufoe furnace under a very slowly flowing unpurified

argon atmosphere and heated for 15 minutes at 1550 °C ± 10°.

The metal shot readily melted and formed an equilibrium-shaped

bead 1.5 cm in diameter. The block and bead were cooled under

argon flow overnight. On removal from the furnace the formerly

white protective coat surface had a deep green color under the

bead and at the bead margin. One to 2 mm from the bead margin

was a purplish blue zone about 1 mm wide.

Direct scans with the electron probe revealed that the outer

purplish blue zone had considerable amounts of chromium. However,

the outer zone had essentially no cobalt but did have manganese.

Green zones,, as expected, were rich in chromium and contained

appreciable amounts of cobalt and very small amounts of iron.

As before, no tungsten, molybdenum, tantalum, or nickel were

found to have entered the investment-coat substrate.

Since manganese oxides are known to produce purple-blue

colors, e.g., in borax glass beads,12 it is assumed that the blue

coloration is due primarily to dissolved manganese oxide. The
-17-

approximate vapor pressures at 1550 °C, in torr , are reported

to be as follows:

Mn = 10 ia > Cu = 10~° * 9 > Cr = 10" 1 * 8 > Fe = 10~ r; >Co, Ni = 10~ r •' -

13

Ti = 10*" 3,9 > Mo = 10“ 9,6 >Wand Ta, both less than 10~ n .

Therefore, it is obvious that electronprobe diffusion results correspond closely to a vapor pressure sequence in which elements in

the alloy with the higher vapor pressures have diffused more than

those with the smaller vapor pressures. The presence of cobalt

in the investment may be attributed mostly to its high concentration

in the alloy, and the absence of nickel to its relatively low

concentration

.

To obtain large amounts of reacted material for X-ray analyses,

two blocks of Synchro-X investment were reacted with Duralliumalloy pellets in a silicon carbide tube furnace. These blocks

were from the previously described series fired at 1050 °C. pne

block was fired with a pellet at 1550 °C ± 20° in a slow, purified

air flow for 15 minutes. Another block was fired under unpurified

argon flow in the same way. At the firing temperatures, alloy

was very fluid and formed well-shaped beads. All beads were cooled

below 1400 °C in an argon flow in less than two minutes. Cooling

was allowed to proceed overnight in the furnace.

The alloy bead was observed to bloat when fired in air and

after a time to become much less fluid. Alloy corrosion in air

firings was, therefore, much more restricted than under argon,
-18-

where the alloy bead remained highly fluid and relatively

unoxidized

.

X-ray powder patterns of various reacted zones in the

investment blocks were obtained. Also, a powder pattern was

obtained from a reaction-product concentrate prepared by

heavy liquid separation of scrapings from an investment used

IV

in casting (Table sal)

.

Reaction products formed in the air fire were eskolaite

(Cr ? Oj , picrochromite (MgCr 2 0 4 ) and cr istobalite . This is

the same as that reported by Allan and Asgar 1

. In the argon

fire, picrochromite and cr istobalite were formed in major

amounts, but only minor amounts of eskolaite were formed.

X-ray examination of the concentrate prepared from cast

investment revealed that picrochromite, eskolaite and

probably cr istobalite were formed.

Polished sections were prepared through reacted layers

at the investment-casting interface. Epi-darkf ield microscopic

examination of these layers revealed that zones with purpleblue internal reflections are commonly to be found slightly

deeper in the investment than those with green internal reflections

(Figures 24 to 26)

.

These purplish-blue zones may owe their

color to dissolved manganese oxide and the green to chromium

oxide in the form of eskolaite.

Very thin sections through reaction layers from spent

investment show a finely submicrocrystalline, sparkly birefringent
-19-

phase which is probably eskolaite. However, in the usual thin

section the alloy-investment reaction products appear opaque

(Figure 27)

.

Mixtures of eskolaite laths and tablets commonly

occur along with picrochromite and blebs of alloy in a vitreousappearing constituent that contains many fine dendrites (Figure

28) . It appears that corrosion in the investment interface is

the result of the oxidation of vapor-transported metal alloy

constituents. Manganese, iron and cobalt oxide (MnO, FeO,

and CoO) tend to drive reactions with silica toward low melting

olivine fields. These oxides# along with the high temperature

and abundant mobilizers in the investment, lead to extended

silica fusion. Fused magnesium oxide combines readily with

chromium to form a refractory magnesium spinel. The chromium

oxide, eskolaite, is also quite refractory up to 1900 °C. Under

low oxygen pressures it appears that reactions of chromium to

form picrochromite are favored. In air, the reactions of chromium

oxide are shifted more toward the formation of eskolaite.

Polished sections reveal that almost all investment pores

along the interface are plugged by vitreous constituents and

crystalline reaction products (Figures 21 to 29)

.

As has been

suggested by Jones14

, it would appear that highly viscous melt

formed during casting which plugged pores to this degree might

greatly cut down gas permeability during casting.
- 20 -

Conclusion : Phosphate investments depend for their high strength

on sinter bonding mostly through fusion-induced silica recrystallization. Iron and other minor non-phosphate constituents

greatly extend the degree of silica fusion. Liquefaction in

the high burnout fire and during casting also induces investment interface irregularity which causes much of the castalloy surface roughness.

Thermal and corrosive effects from casting induce intense

liquefaction in close proximity to the interface. This may,

during casting, (1) cut down the ease of gas escape from

the mold thus promoting casting porosity; (2) result in anisotropic mole/ deformation that could give a poorly fitting cast,

and (3) contribute to cast-alloy roughness by induction of

inter-particulate clumping in the investment.
- 21 -

References

1. Allan, F. C. and Asgar, Kamal. Reaction of cobalt-chromium

casting alloy with investment. J. Dent. Res. _45 (No. 4)

1516-1528, 1966.

2. Durallium Products Corporation. "Manual for Durametric

Technic"

.

3. Frondel, Clifford. "The System of Mineralogy, vol. Ill

Silica Minerals". John Wiley and Sons, Inc., New

York and London, 264-284, 1962.

4. Rogers, A. F. The Occurrence of Cristobalite . Am. J.

Sci. 45 222, 1918; J. Geology 30, 211, 1922.

5. Winchell, Alexander U. and Horace. "The Microscopical

Characters of Artificial Inorganic Solid Substances,

Optical Properties of Artificial Minerals" . Academic

Press, New York, 1964.

6. Rait, J. R. "Basic Refractories, Their Chemistry and

Their Performance." Inter science Publishers, Inc.,

New York 174-192, 1950.

7. Tien, T.-Y. and Hummel, F. A. The system Si0? -P 2 0 F

. J.

Am. Ceram. Soc. <45 (No. 9) 422-424, 1962.

8. Ferguson, J. B. and Merwin, H. E. Art. VII. The Ternary

System Ca0-Mg0-Si0 3

. Am. J. Sci. _48 (No. 284) 81-213

1919.
- 22 -

9. Wentrup, Hanns. Beitrag zum System Eisen-PhosphorSauterstoff, Archiv fttr das Eisenhuttenwesen, 9.

Jahrgang, Heft 1, 57-60, 1935.

10. Tromel, von Gerhard and Schwerdtfeger , Klaus. Untersuchungen

im System Eisen-Phosphor-Sauter stoff . Archiv fur das

Eisenhuttenwesen, 34. Jahrgang, Heft 1, 55-59, 1963.

11. Kreidler, E. R. and Hummel, F. A. J. Inorganic Chem. 6_ (No. 5) 891, 1967.

12. Moses, Alfred J. and Parson, C. L. "Elements of Mineralogy

Crystallography and Blowpipe Analysis." D van

Nostrand Co., 216-218, 1900.

13. Honig, E. W. Vapor Pressure Data for the Solid and Liquid

Elements. RCA Review, Vol. 23, No. 4, 567-586, 1962.

14. Jones, Derek W. Thermal Behavior of Silica and its

Application to Dental Investments, Part 1. Brit.

Dent. Journal, Vol. 122, No. 3, 91-97, 1967.
23 3 X F- a ffi > a cn 3 X ffl F- Q in > o c/3 a

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CO F o o F O o p O o o F o a F F F CDrt O CD F4 • O F 0 1 N O o • l X

rt F4 F • o o F 1 o CD O o F o F O • CD CL F ! • 1 F F F

CD • O A A A CO o o « m o O O X

rt • F O hh • • e F- F 1 O F o o o 3 • 1 o o o QJ FJ F F F rt

CD F A A A CL I £ O O o F 0 • • • X O o O o F' o o o F F F • • O F i-3 F • • 3J 1 QJ 1 O o F F F F • F I F 1 F O O O 1 3 1 5 0 • 0 F F O o D l F OFI

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CD

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rtfo

lQ

QJ *VF-O>P

qjF

co

CDCO
Ill

24

Cone

I

m

vvs

vvs

m-s

s

s

m

vvw

vw

m

m

m-s

vvw

Table II. X-ray Powder Pattern Data on Unfired Investment

70/200

HC1 Digested (XRD-26) Very Magnetic Concentration (XRD-25)

a& Assign. I dA Assign

.

vw-w 5.48 mont

.

vvw 5.16

w 4.82

4.29 qtz

.

w 4.08 mont

.

4.06 crist

.

w 3.86 mont. + forst. ?

m 3.62 mont

.

3.35 qtz

.

w 3.34 qtz

.

3.15 crist. w 3.16 mont.

w-m 2.905 mont.

2.85 crist.

vm 2.73 forst.

m-s 2.65 mont.

w 2.57 mont.

w 2.52 mont. + forst.

2.48 crist.

qtz.

f 2.46 forst.

mont

.

s 2.43

peri.

vvw 2.38 mont

.

vf 2.33

2.31 part crist.

2.27 qtz

.

vw 2.27 part forst.

2.23 qtz

.

vvf 2.20 mont

.

vvf 2.16

2.135 crist.

qtz

.

ss 2.11 per i

.

2.04 qtz.

1.99 qtz.

m 1.95

1.94 crist

.

1.88 crist.

1.82

1.81

qtz

m 1.81 mont.

w 1.75 mont. + forst.

vw 1.705 mont.
Table II. X-ray Powder Pattern Data on Unfired Investment

(cont ' d)

70/200

Cone HC1 Digested (XRD-26) Very Magnetic Concentration (XRD-25)

vvw 1.69 crist.

vw 1.675 qtz

.

vw 1.675 mont

.

vvw 1.649 crist. vf 1.643

vvf 1.613

m 1.612 crist.

m 1.579

w 1.548

w 1.539 qtz

.

vw 1.539

1.536 crist. 1.536

w 1.500 crist.

vws 1.488 peri

.

vvw 1.458 qtz.

vvvf 1.446

vvw 1.429 crist.

vvvf 1.417

vs 1.378 qtZ. IlCTftMB.

vw 1.400

f 1.350

vw 1.340 crist.

vvw 1.307 vvf 1.314

vvw 1.293

w 1.283 crist.

w 1.260 qtz.

qtz = quartz

crist. = cristobalite

mont. = monticellite

for st. = forsterite

peri. = periclase
Table III. X-ray Powder Patterns of Various Fractions

of High-Fired Synchro-X Investment

XRI>- 12 >IRD-24 i XW>-11 Minus-400 mesh

uents obtained

from 100-150 mesh

-car.. : fraction

Minus-400 mesh after plus 70 mesh disaggregation

100-150 aesh

Very magnetic

after plis 70 mesh

Plus-400 mesh Float from Uromoform

Fraction of a 1 separation of minus disaggregated -400 mesh disaggregated

very magnetic 100-150 mesh very tion of XRD-14

100-150 mesh

very magnetic

fraction, uncrushed

I BiAJ Assign. _I tliAJ Assign I 57X1 Assignf 8.37 vw 5.60 SiP.O,"? vw 5 60 vw 5.94 SiPs O? ~? fojtf

f 4.54 trid. f 4.50 trid.

m 5.1’ oliv. w 5.14 forst. w-m 5.20 forst. m 5.11 forst . f 4.75

f 4.63 wa 4.29 qtz. g 4.28 qtz. w 4.3. w 4.30 qtz. w 4.29 qtz. + minor fcarst trid.' ? m 4.27 qtz.

trid. -?

ss 4.07 cnst. 8s 4 ' 07 srs 4.06 crist.

trid-?

88 406 ST8 ,

w-m 3.87 trid.?

to 3.69 SiP; 0-> ~?

ss 3.36 crtz trid 7

w 3.87 trid.?

m 3.46

ss 3.34 'Grid. qtz.

s 3.89 forat. ra 3.90 forst

.

s 3.83 trid.? oliv.

s 3.86 trid.? +

m 3.70 SiP.-O,"? part forst. m-s 3.6* oliv. -? w 3.50 forat.

to 3.41 oliv. m 3 47 part ws 3.3' qtz. f 3.35 o Qtz. ~ J '“ _

vws 3.34 qtz.

*/ 3.21 m 3.15 erist.

f 2 73

v 2.47 qt*.

« 2.285? qtz. w-n 2.24 qtz.

m 2.13 qtz.

w-W 2.08

w 2.03

- a 1 94 part

s 1.82 qtz . wf 1.78 vw 1.75 w 1.70 part w 1.67 qtz.

vw 1 663

vW 1.641 qtz. m 1.612 minor ww 1.563

s 1.543

*-• 1.497 qtz. criat. W 1.482 vw 1.453 qt*. «

w 1.436

V 1.402 —« l- 303 part

1.374 qtz. v 1.352 vw 1 340

vw 3.01 wf 2.92 trid.? m-s 2.86 cnst.

f 2 80 wt 2.74

vf 2.68 inont.-?

vf 2.54 trid.

s 2.49 trid.

w 2.42

m-s 2.28 trid. ? qtz. w-m 2.24 qtz.

m-s 2.13 p. qtz.

w 2.07

TW 2.04 trid.?

w-m 2.02

f 3.19 m-s 3.1b crist. _

ww- 2.69 mont or

ws 2.52 part megnesioOliv.

ws 2.44 pari.

qtz. ouv.

( w 2.35 minor oliv.

vs 2.25 qtz.

m 2.15 oliv.

qtz.

« 2.02 oliv.

,-M 2.98 minor

forst.

mf 2.70 mnt. or

£“8t '

ws 2.52 forst. ws 2.45 pari.

+ forst . mf 2.40 forst. vw 2.36 part fcrst. vw 2.32 part- forst .

s 2.26 forst.

w 2.16 pmrt forst.

TO 2.11 psri. 200

m 3.16 crist.

" 2 ' 97 * 2.86 crist.

m-e 2.78 forst.

crist.

peri.

t 2 . 31

** w 2.28 qtz.

- forst.

in 2.26 forst.

* 2.16 part forst

vw 2.12 qtz.

m 2.11 peri.

m 3.00

s 2.79 forst.

vs 2.53 forst.

s 2.46 qtz.

w 2.37 w 2.33

s 2.27 qtz. forst.

f 2.22 m 2.17 part forst.

vws 2.12 peri. crist.

w 3.15 crist.

m 2.34 crist.

m-s 2.76 forst.

vs 2.51 forst.

s 2.44 qtz + forst

.

ww 2.34 forst.

m-s 2.25 qtz. + forst

.

m 2.11 pen^

minor crist.

* 1.98 QtM . w 1.97 qtz. m 1.93 " 1 95 * 1,M aill. vw 1.95 w-m 1.93 cnst.

, AS Lfi2 6?rat,_ m-s 1.88 cnst mtnnr forst.

s 1.82 qtz. m-s 1.81 qtz. m 1.82 part m 1.82 qtz.

f j 7B ' a 1.75 forst. wvf 1.72 £ U-Zi

w 1.69 f 1.69

. wf 1.70 Crist. f 1.71

a 1.67 qtz. to 1.67 qtz. . X..7 w 1.67 part

qtt. m 1.68 ’ 11,1 q“

nor for . t . w 1.661 m LM3

w 1.638 minor font- W i fti minor forst. w 1.640 —| ejc, -f~T~ f m 1.612 w 1.614 part font , m 1.612 minor

part forst. cnst.

1 T7«l ATTOf

crist

s 1.609

< 1.586 Minor ollv. . m 1.573 vw 1.563 m 1.564 W 1756? w 1.56

s 1.542 qtz. vf OH qtz. 213 m 1.543 part m 1.54 qtz. vw 1.518

vf 1-515 •svf 1.515

£ 1.511

b 1.496 ws 1.494 Enor” ' ws 1.490 peri. m 1.490 fieri, minor forst.Hftnor —« 1.454 w 1.452 £ 1.455 part

Lu!42

" " 45

TT36 “

f 1.434 =,.t. -1.

l L.418 cript^—

m 1.399 m 1.403

m 1.383 a T73B^ 1.383 P»« vw 1 . 306

• 1.373 qtz- vw 1.374 qtz. — 1 - 37 qtn. w 1.353

g 1.346 m 1.349 part w-m 1.348 m-s 1.351 minor m-s 1.35 crist. minor forst.

w 1.302 • 1.290 qtz.

f 1.336 vf 1-322 m 1.310 —Q LJ12 l-Z __

m 1.311 w 1.312 minor

f 1.309

j v-e 1 . 300

1 911*14 w 1.288 si. nor w 1.287

Ollv. - olivlna - -or.tlc.llit. trid . - trldy.lt.
X-ray Pc*^er P»ttem« of Syr.chro-X-Alloy Reaction Products 1550*C Air Pire - 15 min.

(XRD-32)

1550*C Argon Pire-15 Bin.

(XRD-29) Cast Sample

OOLD-4)

1550*C Argon Pi re-15 Bin. (Deep layer)

LffiSrljJ

1550*C Argo 1 Pire- 15 Bin. (Green Shallow Area)

(XRD-30)

-Oil) Aaaigr. mil piXi BtAJ mil

4.81 chran

.

4.07 crist.

3.64 eskol.

3.35 otz. 3.13 crist.

3.04 aCoaPa©??

2.84 crist. 2.67 eskol

2 52 chrom

.

* 4.81

f 4.48

f 4.21 vw» 4.05

3.32

3.13 2.95

2.84 2.77

2.66

2.58 2.51

qtz. crist. chrom

.

crist.

forst. eskol.

chrom.

5.12

4.82 4.47 4.28

4.06

3.88

3.72 3.46

» 3.34

3.24

2.85

2.77

2.67

vwf 2.41 ? chrom.

vwf 2.27 ? eskol.

2.42 chrom. vf 2.34 part forst.

«AA£i.

forst

.

chrom

.

gtz. crist

forst.

min. forst.

crist. minor fora crist.

forst.

eskol. crist.

forst. minor

eskol.

qt.z. minor

part forst. part forst. otz.

forst. part

otz.

vws 4 . 04

3.35

.3.13 3.04

2.94

2.84

aCos Ps O,“?

2.65

2.58

2.43S

4 . 84

f 4.54

f 4.26 wvs 4.06

3.63 3.37

3.15 2.96

2.86

2.78

2.66

2.50

eskol.

gtz.

crist.

forst. •skol.

chrom

.

eskol

.

_J part-

.

2.135 otz. 2.12 crist. 2.11 peri. part

2.09 chrom. 2.08 chrcm. 2.09 chrom. 2.04 eskol. 202

71 T798 otz.

2.03 c ri

1.87 part

Eli®

L

1.87 part

cUIt.

part

. c r*>t. part

C C1I^

1.82 eskol. minor

gtz. vwf 1.73?

1.81 otz.

1.73 vary ainor

vf 1.695 crist. 1.674 eskol.

ICO 1^.574

1.69 pert

1.609 chrom. ainor

crist.

part

otz. minor

eskol

chrom. ®ipor

crist. ainor foxAi-2

chrom

.

»inor

crist.

1.541 qtz.

w-m 1.49b part fors

1.476 chrom. 1.463 eskol. 1.431 eskol.

crist.

1.471 chrom.

1.395 chrom.-?

wf 1.365 crist.

1.428 minor

crist. (?) 1.418 part

crist.

1.397 part chrom.

1.419 part

crist.

1.396 minor forst<.

1.382 part qtz. 1.372 qtz.

1.397 part chrom.

ninor forst.
M

'
Figure 1

Synchro-X powder dispersed in Canada Balsam showing

angular quartz, white birefringent grains (1) , and

blocky cristobalite (2)

;

transmitted light, partly

crossed nicols. X 140
Figure 2

Synchro-X cristobalite grain showing minute disseminated quartz blebs and gas bubbles; transmitted brightfield. X 825
Figure 3

Crushed magnesite grain in Synchro-X powder showing

light-colored monticellite envelopes (1) around dark

periclase spheres (2)

;

transmitted brightfield. X 825
Figure 4

Ammonium phosphate gel-like bond formed on setting

of the mixed Synchro-X investment under a microscope

cover glass; transmitted brightfield. X 140
Figure 5

Thin section through fired Synchro-X investment in

nearly completely polarized light. Angular to subrounded solidly birefringent grains are quartz.

Microcrystalline to submicrocrystalline angular grains

with a spotty birefringent to isotropic appearance are

mostly cristobalite . Compare with same area in following

photograph; transmitted polarized light. X 110
Figure 6

Thin section through fired Synchro-X investment in

slightly polarized light. Angular to subrounded

more birefringent grains are quartz. Light gray,

poorly birefringent areas are mostly cristobalite and

dark, nearly opaque zoned areas (m) are partly fused

calcined magnesite. Same area as the previous photograph;

transmitted slightly polarized light. X 110
;

w<

Figure 7

Polished section of silica-rich region in burned-out

Synchro-X investment consisting of quartz and cristobalite grains (light gray) . Mounting medium occupies

the darker gray areas between the light gray; epibrightfield illumination. X 700
Figure 8

Thin section through quartz grain in Synchro-X

investment not affected by cast-induced thermal baking

showing mobilized margin probably consisting mostly of

tridymite. Transmitted brightfield illumination. X 643
Figure 9

Crushed grain mount of burned-out Synchro-X with

quartz fragment (q) surrounded by low index matrix

material (t) , probably mostly tridymite. Transmitted

brightfield illumination. X 643
Figure 10

Polished section through partly fused magnesite

inclusion (center) in burned-out and fired Synchro-X

investment. Cores of rounded and embayed periclase (light

gray) are rimmed by magnesium-calcium-silicate reaction

product. Epi -brightf ield illumination. X 700
Figure 11

Polished section through partly fused magnesite

inclusion in burned-out Synchro-X investment

showing "liquid bridging" (upper left) to surrounding

mobilized areas. Rounded corroded periclase crystals

are rimmed by fused areas consisting largely of fused

magnesium-calcium-silicate reaction product. Black

irregular areas in fused areas are pores. Epi-brightfield

illumination. X 700
Figure 12

Polished section through fused magnesite inclusion

(light medium gray) in fired Synchro-X investment

showing its corrosion embayments and caries in

silica (dark medium gray), mostly quartz grains.

Very light gray relicts in core of fused area are

probably mostly ferric oxide (hemitite) . Epibrightfield illumination. X 700
Figure 13

Polished section through partly fused deadburned

magnesite grain in burned-out Synchro-X investment

showing a submicrocrystalline calcium-magnesiumsilicate rim about periclase core (solid reddish yellow)

.

The reddish yellow color is due to finely disseminated

ferric oxide. Diffusion into the investment is

delineated by light yellow stringers. Epi-brightf ield

illumination, oil immersion. X 700
Figure 14

Thin section of burned-out and casted Synchro-X

investment probably affected by thermal baking action

around magnesite inclusion with relict periclase

(P , to left of center), ferric oxide blebs (deepest

reddish brown) and liquefied streamers and lobes (R)

penetrating into surrounding porous material.

Transmitted brightfield illumination, oil immersion.

X 643
Figure 15

Thin section through magnesite inclusion in

burned-out Synchro-X investment showing partial reaction of monticellite envelop (bright white) around

rounded periclase crystals (dark gray to black) to form

magnesium-calcium-silicate (lightest gray to left)

.

Partly polarized transmitted light, oil immersion. X 643
Figure 16

Thin section through partly fused magnesite

inclusion in burned-out Synchro-X investment showing

moderately birefringent microcrystalline magnesiumcalcium-silicate reaction product (white) around

periclase and ferric oxide core (black) . Completely

polarized transmitted light, oil immersion. X 643
Figure 17

Thin section through partly fused magnesite inclusion

in burned-out Synchro-X investment showing ferric

oxide rich zone (black lobaceous ring) between

periclase core (dark gray and magnesium-calciumsilicate reaction product rim (lightest gray) . Same

site as in previous figure. Partly polarized transmitted light, oil immersion. X 643
(a)

Figure 18

(b)

Topographic configuration of grain clusters on

surface of Synchro-X investment given different

heat treatments. Identical exposures, epi-darkfield

illumination. X 700 (a) 1050 °C ± 50°; (b) 560 °C ± 10°
Figure 19

Polished section in zone of Synchro-X investment

mobilized by thermal baking resulting from casting.

Fused regions of tridymite, quartz and possible

vitreous material (lightest gray) cary and embay

relict quartz and cristobalite . Darkest gray-toblack areas are mounting medium and unfilled voids

.

Center of field obstructed epi-illumination. X 700
Figure 20

Polished section exhibiting silica mobilization,

center, just below the cast-Synchro-X investment

interface at upper right. Epi-brightfield illumination.

X 700
Figure 21

Polished section showing extended interconnection of

micro-environments of liquefaction below the cast of

Synchro-X interface. Bright gray submicrocrystalline

phases near interface are intermixed chromic oxide and

magnesium-chromium spinel. Epi-brightf ield illumination.

X 515
Figure 22

Polished section through liquefied Synchro-X interfacial region (1) that abutted against alloy cast

exhibiting deep silica mobilization by "liquid-bridging"

into investment body. White blebs in fused region are

alloy, the lightest gray areas consist of chromic oxide

and magnesium-chromium spinel. Solid dark-gray region

along entire left margin is mounting medium. Center of

field obstructed epi-illumination. X 700
Figure 23

Polished section showing limited silica mobilization

beyond Synchro-X-cast interface. Epi-brightfield

illumination. X 515
Figure 24

Polished section through Synchro-X interface with

removed casting possibly indicating penetration of

manganese (blue) deeper into the investment than

chromic oxide (light green) . The manganese oxide

is probably largely part of a vitreous constituent.

Epi-darkf ield illumination. X 643
Figure 25

Polished section through alloy-casting interface of

Synchro-X investment showing purplish-blue zone in

the investment caused primarily by manganese oxide.

Greenish crystals in very thin layer along the interface are mostly eskolaite (Cr 2 0 3 ) . Note that the

casting has been removed. Epi-darkf ield illumination,

oil immersion. X 795
Figure 26

Polished subparallel section through alby-casting

interface with intact casting (dark black at left)

.

Greenish crystals are mostly eskolaite, minute purplishblue regions owe their color probably to manganese oxide.

Epi-darkfield illumination, oil immersion. X 795
Figure 27

Thin section through Synchro-X alloy reaction layer

showing penetration of alloy reaction products,

deep black opaque areas. Investment is to upper

left, and mounting medium filling cast void is to

lower right. Transmitted brightfield illumination.

X 430
Figure 28

Polished section through magnetic concentrate of

Synchro-X cast reaction layer subparallel to the

interface. Note fine dentritic growths in vitreous

region (v)

.

Long bright gray crystals probably are

chromic oxide; more equant bright gray grains are

probably magnesium-chromium spinel. Round very

bright white blebs are alloy. Cloudy milky-white

areas are incompletely reacted quartz and cristobalite. Dark gray areas appear to be largely recrystallized silica. Epi-brightfield illumination. X 515
Figure 27

Thin section through Synchro-X alloy reaction layer

showing penetration of alloy reaction products,

deep black opaque areas. Investment is to upper

left, and mounting medium filling cast void is to

lower right. Transmitted brightfield illumination.

X 430
Figure 28

Polished section through magnetic concentrate of

Synchro-X cast reaction layer subparallel to the

interface. Note fine dentritic growths in vitreous

region (v)

.

Long bright gray crystals probably are

chromic oxide; more equant bright gray grains are

probably magnesium-chromium spinel. Round very

bright white blebs are alloy. Cloudy milky-white

areas are incompletely reacted quartz and cristobalite. Dark gray areas appear to be largely recrystallized silica. Epi-brightfield illumination. X 515
Figure 29

Polished section through Synchro-X interface with

casting showing greenish areas consisting mostly

of submicrocrystalline chromic oxide. Void once

occupied by removed casting is in upper half of

photograph. Blacker regions between the greenish

crystals appear highly vitreous. Epi-darkfield

illumination. X 643