Foamed insulation refractory

A foamed insulating refractory shape is made by: (1) preparing a slip of water, deflocculating agent, refractory aggregate, at least 7.5% by weight cement, and, optionally, clay; (2) forming a foam of water, foaming agent, and air having a density not over about 5 lbs per cubic foot (80 g/l); (3) admixing the slip and the foam to form a foamed slurry with a viscosity between 1000 and 30,000 centipoises, the amount of water in the slip being adjusted to yield the desired viscosity in the foamed slurry; (4) pouring the foamed slurry into molds; (5) curing the pieces so formed at a temperature not over about 72.degree. F. (about 22.degree. C.) for about 16 to 24 hours; (6) drying the pieces at a temperature not over about 200.degree. F. (about 93.degree. C.); and (7) firing the pieces. The method of this invention produces insulating refractory shapes with higher strength-to-weight ratios than those produced by prior art methods.

BACKGROUND OF THE INVENTION 
This invention concerns refractories and particularly insulating 
refractories. 
Insulating refractories are known and are generally refractories with 
relatively high porosities (e.g., 50 volume percent or more). In other 
words, insulating refractory shapes are made by creating holes or voids 
within the shape to provide thermal insulation. 
It is conventional practice today in producing most insulating refractory 
shapes to admix with the refractory material a substance, for example 
sawdust, which will burn out during firing and leave voids. 
It is also known to form insulating refractories by incorporating air, for 
example in the form of a foam, into a slip or slurry of refractory 
particles, for example as set forth in U.S. Pat. Nos. 2,292,011, 
3,232,772, and British Pat. No. 1,124,514. One of the main problems in 
this approach to producing insulating refractories is the stability of the 
foamed slurry. If it is exceedingly unstable, it may even collapse before 
the refractory shape is formed. In any case, it is essential that the 
foamed slurry maintain its structure and not collapse before it has dried 
and formed a semi-permanent, rigid structure. Also, the foamed slurry must 
not crack during the setting and drying steps. 
(In this specification, the term "slip" refers to the mixture of water and 
solid ingredients, with or without a deflocculant, before foaming or 
aeration; the term "foam" refers to the air/water mixture, including a 
foaming agent, often referred to in the industry as a "preformed foam"; 
and the term "foamed slurry" refers to the mixture of "slip" and 
"foam"--sometimes referred to in the industry as "foamed slip"--and also, 
in discussing the background of the invention, to a "slip" which has been 
aerated in situ, for example by whipping in air.) 
One solution to foamed slurry stability is to add an organic binder such as 
starch or polyvinyl alcohol to strengthen the foamed slurry. However, this 
has the disadvantage that foamed slurries containing such organic binders 
require a relatively long drying time, a matter of days, which is 
disadvantageous in mass production. 
The present invention is directed toward the solution of the problem of 
producing a foamed insulating refractory shape which has a high 
strength-to-weight ratio and which is adapted to being made on a mass 
production scale. 
SUMMARY OF THE INVENTION 
It has now been found, according to this invention, that an insulating 
refractory shape of improved strength-to-weight ratio can be made by: (a) 
preparing a slip of water, deflocculating agent, finely divided solid 
refractory particles, and binder; (b) preparing a foam of water, air, and 
foaming agent; (c) admixing the slip and the foam to produce a foamed 
slurry; (d) casting the slurry into molds; (e) curing and (f) drying the 
cast pieces so formed; and (g) firing the pieces, if: (1) the slip 
consists essentially of at least 7.5% cement, from 0 to 50% clay, the 
balance of the solid ingredients being refractory aggregate all of which 
passes a 28 mesh screen, all percentages being by weight and based on the 
total weight of dry solid ingredients, and sufficient water to produce, 
when mixed with the foam, a foamed slurry having a viscosity of from 1000 
to 30,000 centipoises; (2) the foam has a density of not over about 5 pcf; 
(3) the slip and the foam are admixed in the proportion of from 0.25 
volume to 3 volumes of foam for each volume of slip; (4) the cast foamed 
slurry is cured at a temperature of not over about 72.degree. F. (about 
22.degree. C.) for from 16 to 24 hours and then (5) dried at a temperature 
of not over about 200.degree. F. (about 93.degree. C.). 
DETAILED DESCRIPTION 
One way to conceptualize the present invention is to imagine the foamed 
slurry to have been poured into a mold one foot on a side, forming a block 
of one cubic foot volume. The question to be considered is: What are the 
relative amounts of solid material, water, and air in this cubic foot? 
Since air has substantially no weight and the water will be removed upon 
drying, the weight of the finished refractory shape (in other words, its 
density in pounds per cubic foot--pcf) is attributable to the solid 
materials in the foamed slurry. Since, in many cases, some of these solids 
will lose weight, due to loss of water or other components, during the 
firing operation, the weight of the solid ingredients in the foamed slurry 
will not be exactly equal to the weight (per cubic foot) of the fired 
refractory, but it is well within the skill of one versed in the art to 
make the necessary calculations, given the characteristics of the raw 
materials to be used and the desired density for the final fired 
refractory shape. Also, it will be understood that usually the cast foamed 
slurry will undergo drying and firing shrinkage and that these must be 
taken into account in relating the foamed slurry density to the desired 
density of the fired product. Generally, insulating refractories produced 
by this method will have final fired densities ranging from as low as 20 
pcf (0.3 g/cc) up to 100 pounds pcf (1.6 g/cc), or even higher, although 
this is generally considered the upper limit of "insulating" refractories. 
In any case, the choice of the density of the finished refractory is up to 
the producer. 
Having determined the amount of solid materials in the cubic foot of foamed 
slurry under consideration, the next question is: How much water should be 
present? This leads to the first discovery of the present invention: In 
order to achieve maximum stability in the foamed slurry, the amount of 
water in it should be kept to a minimum, consistent with producing a 
foamed slurry having a viscosity of between 1000 and 30,000, preferably 
between 5000 and 12,000, centipoises. This viscosity range results from 
the fact that, while the water content must be held to a minimum, the 
foamed slurry must be of low enough viscosity so that it can be poured 
into molds. 
It will be understood that lower viscosity implies more water in the foamed 
slurry. It has been found that denser foamed slurries (i.e., those with 
high solids content) can tolerate more water without collapsing. Thus, the 
broad range of viscosities given covers foamed slurries of different 
solids contents. The narrower range given (5000 to 12,000 centipoises) is 
the preferred operating range for all foamed slurries. When operating near 
the upper end of the viscosity range (near 30,000 centipoises) it may be 
necessary to mechanically place the foamed slurry in the molds. 
One of the interesting sidelight discoveries of the present invention is 
that the optimum amount of water used per cubic foot of foamed slurry is 
roughly constant no matter how much solid material is in the cubic foot, 
being about 0.18 (.+-. about 20%) cubic foot or about 11 (.+-.2) pounds 
per cubic foot of foamed slurry (0.18.+-.0.03 g of water per cc of foamed 
slurry). Since the amount of water per cubic foot of foamed slurry is 
approximately constant, it will be evident that the amount of water per 
weight of solid material will be less in a foamed slurry, and hence also 
in the underlying slip (see discussion below), used to make higher density 
refractories. This is completely contrary to normal slip casting 
procedures, where it is assumed that a single, optimum amount of water 
exists which will be "best" for any given slip. In other words, the slips 
used in this invention are not necessarily made to have the optimum 
deflocculation, as is done in slip casting. 
Once the total amount of water to be used has been determined, the question 
remains: How should this amount of water be divided between the slip and 
the foam? First, it may be pointed out that all the water could be placed 
in the slip, which would mean that the foam would have to be formed in the 
slip itself, for example by whipping the slip or by generating a gas in 
it. While this method is known, it has several disadvantages, the main one 
of which is that it is exceedingly time and energy consuming; also, it is 
difficult to control the final density of the refractory since there is no 
direct measure of the amount of air placed in the slip. It is also 
difficult to obtain, by this method, a foamed slurry of uniform 
characteristics. Second, at the opposite extreme, it is possible to place 
all the water in the foam, thus adding dry solid ingredients to the foam. 
Again, this method is known, but it results in forming a very poor foamed 
slurry, the solids tending to ball up and agglomerate rather than 
distributing themselves evenly throughout the water films of the foam. The 
net result is a very weak final product. 
Having concluded that some of the water should be in the slip and some in 
the foam, we come to the second discovery of this invention: The strongest 
refractories are formed when the water in the foam is kept to a minimum. 
This means the foam will contain less than 5, preferably less than 3, 
pounds of water per cubic foot of foam (less than 80, preferably less than 
48, g/l). A density of 2 pcf (32 g/l) is a reasonable value for the 
density of the foam. While lesser amounts of water could be used in the 
foam, such foams tend to be unstable and as a practical matter it will 
prove very difficult to form foams of less than 1 pcf (16 g/l) density. 
While the preceding conceptualized discussion of a single cubic foot of 
foamed slurry may aid in understanding the invention, the producer of an 
insulating refractory shape wants to know how to proceed step by step in 
his manufacturing operation. 
From this point of view, the first step is the formation of the slip. The 
slip will contain the dry ingredients and, based on the total weight of 
dry ingredients, from about 30 to 40 weight percent water, the exact 
amount of water being determined, as set forth above, by that necessary to 
achieve the specified viscosity in the final foamed slurry. It will 
generally be desirable that the slip have as low a viscosity as possible, 
so that it will mix readily with the foam, and accordingly it will 
customarily contain a deflocculating agent, as is well known in the art. 
The principal solid ingredient will be refractory aggregate. This may be 
any one or more of refractory grog, kyanite, calcined clay, bauxite, 
alumina, or any other refractory aggregate. The exact composition chosen 
will be dictated primarily by the refractoriness desired in the finished 
product, as will readily be understood by those skilled in the art. There 
is no limitation on the amount of any type of aggregate; for example, the 
aggregate may be all calcined flint clay. In any case, the refractory 
aggregate will all pass a 28 mesh screen. The finer the aggregate the 
stronger the resulting shape, but coarser aggregates yield better thermal 
shock resistance. 
The slip may contain up to 50 weight percent, based on the total weight of 
the dry ingredients, of clay. 
An extremely important dry ingredient is the binder, which must permit fast 
drying of the cast foamed slurry, not interfere with the stability of the 
foamed slurry (i.e., must not cause it to collapse), while at the same 
time providing strength in the dried shape. This leads to the third 
discovery of the present invention: That to achieve these objectives the 
cement must be an inorganic cement such as Portland cement or calcium 
aluminate cement. More specifically, the binder can not be an organic 
material such as starch, gum, polyvinyl alcohol and the like. These cause 
extremely slow drying. 
The choice of cement depends on the refractoriness desired in the final 
product, Portland cement being the least refractory of the cements 
mentioned and high alumina calcium aluminate cement, such as that sold by 
the Aluminum Company of America under the trade name "CA-25," being the 
most refractory. It has been discovered that there must be present at 
least 7.5 weight percent, based on the total weight of dry ingredients, of 
the cement; 10% has been found to be a reasonable amount. While there must 
be a certain minimum amount of the cement present in order to provide 
adequate strength in the foamed refractory, there is no upper limit. While 
it would be possible to make a foamed refractory entirely of cement, such 
a procedure is not very practical because of the poor refractoriness of a 
shape made entirely of cement, not to mention its expense. Accordingly, 
about 30% cement will be found to be a practical upper limit. 
The foam may be prepared in a planetary mixer such as a Hobart mixer or in 
a foam generator, a standard article of commerce, using air, water, and 
foaming agent, as is well known in the art. The only special requirement 
for this invention is that the foam be prepared with a minimum amount of 
water: it will have a maximum density of about 5 pcf (80 g/l). Since the 
foam should be fairly uniform in structure, a density of 1 pcf (16 g/l) 
will be a practical minimum. Foams of about 2 pcf (32 g/l) density have 
been found to work quite well in the practice of this invention. 
The slip and the foam can be mixed in any of various standard pieces of 
equipment, for example a V-blender, a plaster mixer, a paddle mixer, or a 
planetary mixer. It will be evident that the relative amounts of slip and 
foam used will depend on the density desired in the finished product, less 
foam being used when a higher density product is desired. The exact 
proportions needed to achieve any given density will depend on the 
specific gravities of the solid materials used. However, in general the 
proportions of the two will range from 0.25 volume to 3 volumes of foam 
for every volume of slip. 
The foamed slurry is then poured into molds where it will be cured for from 
16 to 24 hours at a temperature not exceeding 72.degree. F. (20.degree. 
C.). In fact, the foam and foamed slurry should at no time be allowed to 
reach a temperature above that specified. Higher temperatures decrease the 
stability of the foam and foamed slurry, causing premature collapse. The 
molds may be made of any suitable material, for example metal or cardboard 
or plastic. Preferably they are arranged so that at least the sides can be 
removed from the cast shapes after the 24 hour curing. 
After curing, the shapes are dried. This may be at an elevated temperature, 
not greater than about 200.degree. F. (about 93.degree. C.). It has been 
found that denser foamed slurries can be dried at higher temperatures than 
lighter ones. For example, a foamed slurry designed to produce a final 
product of 30 pcf (0.48 g/cc) density should be dried at no more than 
110.degree. to 120.degree. F. (43.degree. to 49.degree. C.), at which 
temperature it will dry within 24 hours, while a foamed slurry designed to 
produce a product of 60 pcf (0.96 g/cc) density can be dried at 
160.degree. to 170.degree. F. (71.degree. to 77.degree. C.), within 16 
hours or less. At this point the shapes will be completely dry and of 
adequate strength to be placed in a kiln or, for example, on a car which 
will carry them through a tunnel kiln for firing. 
The exact firing temperature will depend on the solid ingredients used. As 
is well known in the art, refractories of higher Al.sub.2 O.sub.3 content 
will generally be fired at higher temperatures. Examples of specific 
firing temperatures are given in the following examples. 
It may be noted that the solids used in the practice of this invention have 
a density of about 180 pcf (2.9 g/cc); therefore, a density of 60 pcf 
(0.96 g/cc) in the final fired insulating brick means that the brick is 
two-thirds, or 67 volume percent, pores. Similarly, a fired density of 30 
pcf (0.48 g/cc) means that the refractory is five-sixths, or 83 volume 
percent, pores.

As will be evident from the following examples, the method of this 
invention permits the forming of an insulating refractory shape which, 
although it has less than half the density of a "dense" fireclay 
refractory (normally about 10 to 20 volume percent pores), exhibits a 
cold-crushing strength equal to that of the "dense" refractory. 
EXAMPLES 
In Table I are shown several mixes designed to be used at operating 
temperatures up to 2600.degree. F. (about 1430.degree. C.). In each case, 
the slip was prepared from 50 parts by weight calcined fireclay grog 
containing 40% Al.sub.2 O.sub.3, substantially all of the grog passing a 
28 mesh screen and about 80% passing a 200 mesh screen, 20 parts by weight 
plastic fireclay, 20 parts by weight calcined alumina of the type sold by 
Kaiser Aluminum & Chemical Corporation under the name C5R, and 10 parts by 
weight of the calcium aluminate cement sold by Aluminum Company of America 
under the name CA-25, together with the indicated percent (by weight, 
based on the total weight of dry ingredients) of water and 0.1 part by 
weight sodium citrate deflocculent. From the chemical analyses of these 
ingredients, it was determined that the fired mixes would have 
approximately the following chemical analysis: 55.4% Al.sub.2 O.sub.3, 
38.2% SiO.sub.2, 1.6% Fe.sub.2 O.sub. 3, 1.4% TiO.sub.2, 2.2% CaO, 0.2% 
MgO and 1.0% alkali (Na.sub.2 O, K.sub.2 O, etc.). The different amounts 
of water were used in order to achieve different viscosities in the final 
foamed slurries. 
The foam used in each mix was prepared in a planetary mixer, Model N50, 
sold by Hobart Mfg. Co., using 4% by weight of the unaerated liquid (water 
plus foaming agent) of Mearecel 3503, sold by Mearl Co., as foaming agent. 
In each case the foam had a density of 2 pcf (32 g/l). 
One volume of the slip was admixed with the volumes of foam indicated in 
Table I in a planetary mixer for about 15 minutes. The viscosity given in 
Table I is for the foamed slurry and is in centipoises (cps). The foamed 
slurries were then cast into molds 12" (30 cm) on a side and 4 to 5" (10 
to 13 cm) deep. The molds were epoxy coated aluminum sides set on a 
plastic film base. After the cast pieces had cured in the molds overnight 
at room temperature (about 20.degree. C.) the sides of the molds were 
removed and the pieces left on the thin plastic sheet substrate at ambient 
conditions for another 5 days. The pieces were then removed from the 
substrates and fired to cone 16 (about 1450.degree. C.). The various 
pieces exhibited drying shrinkages of approximately 2% and firing 
shrinkages of approximately 7.5%. 
TABLE I 
__________________________________________________________________________ 
Linear 
H.sub.2 O 
Vol Viscosity 
Fired 
Density 
Change 
MOR CCS 
Mix 
(%) 
Foam 
(cps) 
(pcf) 
(g/cc) 
(%) (psi) 
(kg/cm.sup.2) 
(psi) 
(kg/cm.sup.2) 
__________________________________________________________________________ 
1 38.8 
2.0 8500 29 0.47 -0.6 294 
20.7 327 
23.0 
2A 38.8 
1.3 4000 39 0.63 -0.7 293 
20.6 1029 
72.3 
B 36.6 
1.3 4000 38 0.61 -0.8 295 
20.7 989 
69.5 
C 33.3 
1.3 5400 39 0.63 -0.6 300 
21.1 1118 
78.5 
D 30.7 
1.3 7800 41 0.66 -0.6 306 
21.5 1413 
99.4 
3A 33.3 
1.0 3400 45 0.72 -0.4 363 
25.5 1738 
122 
B 32.0 
1.0 9000 47 0.75 -0.5 382 
26.8 1694 
119 
C 30.7 
1.0 5200 47 0.75 -0.3 456 
32.0 2377 
167 
D 29.3 
1.0 11000 
49 0.79 -0.4 413 
29.0 2061 
145 
4A 30.7 
0.7 2200 60 0.96 -0.5 740 
52.0 3752 
264 
B 28.0 
0.7 5600 61 0.98 -0.4 754 
53.0 3123 
220 
C 26.7 
0.7 7600 63 1.01 -0.2 770 
54.0 4416 
310 
D 25.3 
0.7 11000 
64 1.03 -0.4 851 
59.8 4200 
295 
__________________________________________________________________________ 
The fired pieces were cut into brick 9 by 4.5 by 2.5 inches 
(23.times.11.5.times.6.4 cm) on a side, and these brick subjected to 
various tests with the results indicated in Table I. 
The fired density was determined by weighing the brick and measuring their 
dimensions. The linear change was determined after heating the fired 
bricks, at a rate of 400.degree. C. per hour, to 1400.degree. C., holding 
at that temperature for 24 hours, and cooling to room temperature. The 
modulus of rupture (MOR) was determined on the full brick at room 
temperature by ASTM C93-67 in three point loading (7 inch--about 18 
cm--span). The cold crushing strength (CCS) was also determined by ASTM 
Method C93-67 at room temperature on specimens about 4.5 by 4 by 2.5 
inches (11.5.times.10.times.6.4 cm) cut from the broken MOR specimens. The 
brick made from Mix I had a thermal conductivity of 0.92 Btu-in/.degree. 
F.-hr-ft.sup.2 (0.13 watts/m.degree. C.). 
The preceding results can be compared with the properties of a superduty 
fireclay brick having a bulk density of 145 pcf (2.3 g/cc) (about 11% 
porosity). A typical brick of this type exhibits linear change upon reheat 
to 1500.degree. C. of from -0.2 (i.e., shrinkage) to +0.5% (i.e., 
expansion), has a modulus of rupture from around 1000 to 1500 psi (70 to 
100 kg/cm.sup.2) and cold crushing strengths of from 2000 to 3500 psi (140 
to 250 kg/cm.sup.2). Its thermal conductivity is about 8 Btu-in/.degree. 
F.-hr-ft.sup.2 (1.1 watts/m.degree. C.). Thus, it can be seen that a mix 
such as 4 A, B, C, or D according to this invention has cold crushing 
strengths exceeding that of the super-duty brick, a modulus of rupture 
approaching that of the super-duty brick, and yet, because of its much 
lower density, should have less than a quarter the thermal conductivity 
(about 1.8 Btu-in/.degree. F.-hr-ft.sup.2 or 0.26 watts/m.degree. C.). The 
heat savings to be realized in a furnace constructed with bricks such as 
those made from mixes 4 A, B, C, and D are obvious. 
Another comparison of the brick according to this invention can be made 
with conventional insulating brick made with a sawdust burnout material. 
Such brick typically have a bulk density of 50 pcf (0.80 g/cc), a linear 
change on heating to 1400.degree. C. of -0.2%, modulus of rupture from 200 
psi (14 kg/cm.sup.2), cold crushing strength of 200 psi (14 kg/cm.sup.2), 
and thermal conductivity of 1.45 Btu-in/.degree. F.-hr-ft.sup.2 (0.21 
watts/m.degree. C.). Thus, from this point of view, brick made according 
to the present invention can have densities and thermal conductivities 
equivalent to conventional 2600.degree. F. (1430.degree. C.) insulating 
brick, but twice their modulus of rupture and about ten times their cold 
crushing strength. 
Set forth in Table II are various mixes designed for use at operating 
temperatures up to 2300.degree. F. (about 1260.degree. C.). These mixes 
illustrate the use of different types and amounts of solid raw materials. 
Aggregate A is the pulverized fireclay grog used in the mixes of Table I 
and Aggregate B is the same material ballmilled so that 90% passed a 325 
mesh screen. Clay D is the same pulverized plastic fireclay used in the 
mixes of Table I, whereas Clay E is a ballmilled semi-plastic Missouri 
fireclay, and Clay F is an air floated kaolin clay. Cement H is the same 
CA-25 cement used in the mixes of Table I, Cement J is a lower purity 
calcium aluminate cement sold by Universal Atlas under the name Refcon. 
Cement K is an even lower purity calcium aluminate cement sold by 
Universal Atlas under the name Lumnite, and Cement L is an equivalent 
cement sold by Lone Star Lafarge under the name Fondu. 
TABLE II 
__________________________________________________________________________ 
Drying 
Aggregate 
Clay Cement Vol Viscosity 
Shrinkage 
Mix 
Type 
Amt 
Type 
Amt 
Type 
Amt 
Deflocc 
Foam 
(cps) 
(%) 
__________________________________________________________________________ 
5 A 70 F 20 J 10 0.15 2.5 6200 5.6 
6 B 80 E 10 H 10 0.1 2.0 9800 2.2 
7 A 70 F 20 L 10 0.15 2.0 11000 
3.2 
8 A 60 D 20 K 20 0.5 2.0 8600 3.0 
__________________________________________________________________________ 
TABLE III 
__________________________________________________________________________ 
T.sub.f 
Shrinkage 
Density 
MOR CCS Linear 
Mix 
(.degree.C.) 
(%) (pcf) 
(g/cc) 
(psi) 
(kg/cm.sup.2) 
(psi) 
(kg/cm.sup.2) 
Change (%) 
__________________________________________________________________________ 
5 1265 
8.6 27 0.43 
92 
6.5 167 
11.7 -0.2 
6 1290 
9.8 33 0.53 
-- -- -- -- -- 
7 1265 
9.9 35 0.56 
212 
14.9 464 
32.6 -0.1 
8 1290 
9.3 33 0.53 
-- -- -- -- -- 
__________________________________________________________________________ 
The indicated weight portions of the different ingredients were blended 
with 38.8% water (by weight, based on the total weight of dry ingredients) 
and the indicated percentage of sodium citrate as deflocculant to form the 
slip. One volume of this slip was mixed with the indicated volume of the 
same two pcf foam used in the mixes of Table I. The viscosity, in 
centipoises, of the foamed slurry is indicated in Table II. 
The foamed slurries were cast into the same molds used in the examles of 
Table I and subjected to the same curing and drying treatment. The drying 
shrinkages given in Table II are averages over both vertical and 
horizontal dimensions of at least two pieces. 
After firing to the temperatures indicated in Table III, the pieces had the 
properties indicated in that Table. 
In the specification and claims, percentages and parts are by weight unless 
otherwise indicated, except that porosities are expressed in volume 
percent. Mesh sizes referred to herein are Tyler standard screen sizes 
which are defined in Chemical Engineers' Handbook, John H. Perry, 
Editor-in-Chief, Third Edition 1950, published by McGraw Hill Book 
Company, at page 963. For example, a 200 mesh screen opening corresponds 
to 74 microns. Analyses of mineral components are reported in the usual 
manner, expressed as simple oxides, e.g. Al.sub.2 O.sub.3 and SiO.sub.2, 
although the components may actually be present in various combinations, 
e.g. as an aluminosilicate. Cone numbers used refer to the Standard 
Pyrometric Cones (manufactured by Edward S. Orton Ceramic Foundation) used 
to measure the combined effect of time and temperature in the firing of 
ceramic products. Thus, for example, "cone 16" represents a heating to 
2651.degree. F. (1450.degree. C.) at a rate of 108.degree. F. (60.degree. 
C.) per hour which is equivalent to a heating to 2683.degree. F. 
(1470.degree. C.) at a rate of 270.degree. F. (150.degree. C.) per hour. 
Viscosities were measured on a Brookfield rotational viscometer Model RVT 
at 5 rpm.