Kiln furniture for the firing of ceramic articles

Low mass kiln furniture for supporting ceramic ware during the firing thereof is provided having excellent thermal shock resistance and high temperature stability, the kiln furniture being comprised of a porous refractory ceramic body of low density prepared by the firing of a porous aluminosilicate-bound aggregate of refractory ceramic compounds, and wherein the ceramic body has on one or more of its surfaces a smooth skin in the form of an integral thin porous ceramic membrane layer.

BACKGROUND OF THE INVENTION 
The present invention relates to kiln furniture and, more particularly, to 
low mass, porous refractory ceramic articles useful as kiln furniture. 
Kiln furniture refers to the refractory shapes used to support ceramic ware 
during the firing process and includes shelves, contoured supports and 
spacers which are referred to in the industry as batts, setters (box, 
plate, yoke-ring) and the like. These refractory shapes must not only be 
able to withstand the high temperatures encountered in firing ceramics, 
but must also be strong enough to support the necessary load of the 
ceramic at firing temperature without deforming. Also of great importance, 
the kiln furniture must be reusable, a property demanding highly thermal 
shock resistant material. 
Recent advances in high speed or fast-firing kilns in the ceramics industry 
have led to the possibility of improved economics in the manufacture of 
ceramic ware as a direct result of potential for fuel reduction during the 
firing cycle. In order to take maximum advantage of these potentially 
available process cost reductions, however, it is necessary to employ kiln 
furniture which is of low mass and yet which possesses extremely high 
thermal shock resistance in order to withstand the thermal cycling which 
characterizes the newer kiln designs and firing processes. Added to this 
is the need to provide kiln furniture of a variety of shapes and 
configurations as needs may dictate, as well as furniture having smooth 
surfaces suitable for supporting shapes to be fired into ceramics. 
To date, a cost-effective kiln furniture article meeting all these 
requirements has not been realized. 
SUMMARY OF THE INVENTION 
According to the invention, there is provided a lost cost, low mass 
refractory article for use as kiln furniture which possesses the requisite 
thermal shock resistance and smooth surfaces. The article comprises a 
porous refractory ceramic article made by firing a supportable molded 
porous shape comprised of a porous, aluminosilicate-bound aggregate of 
refractory ceramic materials. The fired porous refractory article 
possesses smooth surfaces at predetermined surfaces thereof by reason of 
arrangement there of an integral thin porous ceramic membrane layer. 
In the invention, a foamable ceramic composition containing refractory 
ceramic compounds, components which result in formation of an 
aluminosilicate hydrogel binder, and components which result in in situ 
generation of gas to achieve porosity and low density, is cast or 
injection-molded or extruded into a desired configuration, and in the mold 
or extrusion chamber foams and sets to a supportable porous shape in the 
desired configuration. Included in this process or shortly thereafter, 
steps preferably are taken to provide on selective surfaces a precursor of 
the eventually desired thin porous ceramic membrane in the fired article. 
Prior to firing, the supportable porous shape is treated to reduce the 
alkali metal content thereof so as to increase refractoriness in the fired 
article. Thereafter, the porous shape is fired to produce the low-mass, 
porous, refractory ceramic article preferably having on selected surfaces 
an integral thin porous ceramic membrane layer, these properties and 
features making the article ideally suited for use as kiln furniture. 
The ability to utilize a moldable composition has the distinct advantage of 
permitting economic manufacture of kiln furniture of any desired size and 
shape. The particular composition itself is based upon low cost materials, 
thereby materially enhancing the economics of the process and ultimate 
product, yet nevertheless is capable of producing low-density, porous 
refractory ceramic articles having excellent resistance to thermal shock. 
For example, by virtue of rapid and economic techniques used for reducing 
the alkali metal content of the supportable porous shape, it is possible 
to use large amounts of low-cost alkali metal forms of compounds in the 
composition, while still producing a fired ceramic of low alkali metal 
content and, hence, increased refractoriness and thermal shock resistance. 
So too, by virtue of expedient and economic techniques for producing an 
integral and smooth thin porous ceramic membrane layer on selected 
surfaces of the article, articles especially suitable for kiln furniture 
are producible at low cost. 
According to the present invention, there is provided a composition 
comprised of an admixture of an aluminosilicate hydrogel and suitable 
refractory ceramic materials, e.g., refractory oxides, carbides, nitrides, 
borides, silicides and the like such as alumina, chromia, zirconia, 
magnesia, titania, silica and mixtures thereof (either as admixtures per 
se or as part of the compound itself, e.g., mullite, cordierite, calcined 
kyanite and the like), silicon carbide, silicon nitride, boron carbide, 
boron nitride and the like. Also included as part of the composition are 
particulate metal, a surfactant system and a gel strengthening agent, 
preferably silica fume, these latter ingredients being present in 
essential yet relatively minor proportions relative to the hydrogel and 
ceramic components. In addition, refractory fibers may be included in the 
composition to attain yet additional strength in the eventual molded and 
fired ceramic. 
The foregoing composition is described with reference to generally 
identifiable constituents of the composition at the time of its molding 
and setting, but the general process of preparing the composition utilizes 
more fundamental components which, upon admixture, result in the formation 
of the described hydrogel (i.e., rather than addition of the hydrogel as 
an identifiable separate ingredient). The hydrogel is formed from a water 
soluble source of silica and a water soluble source of alumina, and the 
remaining components of the castable composition (e.g., refractory ceramic 
materials, surfactants, gel strengthening agent, metal powder, refractory 
fibers) can be added to or distributed between one or both of the 
aluminate or silicate compositions. Upon admixture of these two separately 
prepared and maintained aqueous compositions or slurries, there is formed 
an aluminosilicate hydrogel which serves to bind together all components 
of the composition. The hydrogel binder is self-setting at ambient 
conditions and is capable of setting and binding the composition to a 
generally self-supporting structure within a brief but controllable time. 
According to this aspect of the present invention, the component parts of 
the moldable ceramic composition are admixed to form a moldable 
composition. Before any substantial self-setting of the composition 
occurs, it is poured or injected into a mold of suitable desired shape so 
as to assume the general configuration thereof, and/or extruded in the 
desired shape, taking into account the fact that the composition will foam 
and expand in the mold or extrusion chamber. During the self-setting 
reaction, additional reaction takes place within the composition in which 
the particulate metal reacts with alkali metal compounds in the 
composition to produce, inter alia, hydrogen gas. By arranging the 
self-setting hydrogel reaction to be of suitable duration, the cast 
composition increases in volume in the mold or chamber as a consequence of 
the internal gas generation and takes on a porous nature as the gas 
evolves within and from the composition. Then, as close as possible to the 
cessation of gas evolution, the in-situ hydrogel formation causes the 
composition to set in the desired porous configuration, i.e., with a 
porosity that is either predominantly open celled or predominantly closed 
celled. 
The self-supporting structure formed in this manner requires further 
treatment in order to leach alkali metal therefrom so as to render the 
structure suitably refractory and thermal shock resistant for intended use 
in the high temperature and/or temperature cycling environments to which 
kiln furniture is exposed. Preferably this will be accomplished in a two 
step process involving rinsing of the part in water to remove excess 
alkali metal compounds therefrom, and then contacting the part with an 
ammonium salt solution, e.g., a dilute ammonium chloride ion exchange 
solution, whereby ammonium ions are exchanged for alkali metal ions. 
Thereafter, the structure is dried to remove water therefrom, and is then 
fired at suitable conditions to result in the ultimately-desired shaped 
porous ceramic kiln furniture article. 
Porous ceramic articles made from these compositions preferably are 
arranged to possess a smooth thin ceramic membrane layer on all or parts 
of their surfaces by a number of process and/or compositional techniques, 
including the expedient of providing mold surfaces with ceramic paper 
prior to introduction into the mold of the castable composition, and the 
use of defoaming agents on selected mold surfaces to suppress foaming of 
the castable composition at those surfaces and arrive at controllably 
small size pores at those surfaces.

DETAILED DESCRIPTION OF THE INVENTION 
The aluminosilicate hydrogel portion of the composition of the present 
invention is, in essential respects, as described in commonly-assigned 
U.S. Pat. Nos. 4,357,165 and 4,432,798, both of which are expressly 
incorporated herein by reference. As described in those patents, the 
hydrogel results from the admixture of water soluble sources of both 
silicate and aluminate (typically, sodium silicate and sodium aluminate), 
which admixture then self-sets at ambient temperatures in times which can 
be exceedingly short (e.g., on the order of as little as a few seconds but 
typically on the order of a few minutes), but nevertheless can be 
controlled by predetermined choice of molar ratio between aluminate and 
silicate, concentration of water, and temperature. The ability to exercise 
control over setting times for the hydrogel binder leads to important 
advantages with respect to attainment in the present invention of molded 
ceramic kiln furniture articles of both desired geometry and desired 
porosity. Also described in the above-noted patents is the utilization of 
the hydrogel components along with granular refractory particles to 
produce, e.g., molds, by virtue of the self-setting hydrogel serving to 
bind the granular materials into a self-supporting structure. 
According to the present invention, the separately prepared and admixed 
components for forming the aluminosilicate hydrogel have added to them 
and/or distributed between them the remainder of the components which will 
make up the ceramic composition and the eventual fired porous ceramic 
shaped article. As earlier noted, the essential elements of this 
composition, besides the hydrogel-forming constituents, are refractory 
ceramic materials, particulate metal powder, a gel strengthening agent 
such as silica fume and a surfactant component, with refractory fibers or 
other conventional materials optional. The refractory ceramic materials 
generally will be present in the overall composition in a weight 
percentage of from about 50% to about 90%, preferably from about 60% to 
about 70%. In a preferred embodiment of the invention, the ceramic 
materials included in the composition will be chosen from cordierite, 
calcined kyanite and mixtures thereof, with most preferred compositions 
containing nearly equal weight proportions of both cordierite and calcined 
kyanite, e.g., from about 30 to 35% of each ceramic. 
According to the invention, the requisite porosity in the final ceramic 
article to achieve low density kiln furniture is provided as a consequence 
of in situ reaction between metal powder and alkali compounds (e.g., 
sodium hydroxide) present in the composition, resulting in development of 
hydrogen gas as a reaction by-product. As a consequence of this internal 
gas production and evolution, the composition will expand in volume in the 
mold or extrusion chamber and develop porosity, the quantity of 
composition employed obviously being regulated to take into account the 
expected (and predetermined) degree of expansion within the mold or 
chamber to arrive at the desired final density and size of the article. At 
the same time, the surfactant present in the composition serves to break 
up the bubbles of evolving gas in the aqueous composition to achieve, 
controllably, suitably small bubbles and to dictate whether the porosity 
developed in the structure will be of the open-celled type or of the 
closed-cell type as may be desired. 
The preferred particulate metal is aluminum, although other metals or metal 
alloys such as silicon or ferrosilicon which similarly will react with 
alkali compounds present in the composition to produce hydrogen gas also 
can be employed. 
For most generalized compositions, the amount of surfactant and metal (e.g. 
aluminum) powder will be relatively small compared to the other components 
of the system, with the typical levels of addition of the surfactant being 
in the range of from about 0.05 to 1.0 percent by weight of the total 
composition and the metal powder being in the range of from about 0.05 to 
0.5 percent by weight of the total composition. Preferred ranges of 
addition for these materials are 0.4 to 0.8 percent by weight for the 
surfactant (most preferably about 0.6%) and 0.1 to 0.2 percent by weight 
for the metal powder (most preferably about 0.15%), and a preferred ratio 
between the surfactant and metal powder is generally from about 2:1 to 
8:1, most preferably about 4:1. 
Among the preferred class of surfactants (which may be used alone or in 
combination) for use in the invention are the silicone glycols such as are 
available from the Dow Chemical Company for use in producing polyurethane 
foams. These surfactants have a stabilizing effect on the gaseous 
by-products produced and are available in a variety of customized 
formulations (based upon the silicone glycol chemistry) that are designed 
to control bubble (or cell) size as well as to control or dictate whether 
the cells are mostly open or mostly closed. For example, the surfactants 
from Dow Chemical known as DC 190, DC 198, Q2 5160 and Q2 5125, provide a 
mostly open cell structure in the present invention, while other 
surfactants from Dow Chemical, such as DC 193, DC 197, DC 5103 and DC 
5098, provide a mostly closed cell structure in the present invention. In 
addition, still other Dow Chemical silicone glycol surfactants are 
available to further customize a foamed cell structure with a controlled 
or limited amount of cells opened, such as Q2 5243, DC 5043, Q2 5169, X2 
5256, X2 5258 and Q2 5244. Although the silicone glycol type surfactants 
are preferred, a variety of other non-silicone surfactant types also may 
be employed, such as those available from Air Products & Chemicals, Inc. 
under tradename LK-221 and LK-443. 
With respect to the aluminum or other particulate metal, the average 
particle size of the powder employed generally will be in the range of 
from about 1 to 44 .mu.m, and preferably about 6-9 .mu.m, with the 
understanding that the larger the surface area of the metal present in the 
composition, the more vigorous and extensive will be the foaming reaction. 
Another essential ingredient of the composition of the invention is a gel 
strengthening agent, preferably silica fume, although other suitable 
agents may be employed. Silica fume is a by-product collected in the 
airstream during the reduction of silica sand by coal or coke in an arc 
furnace to make metallurgical-grade silicon metal. The particulates are 
hollow spheres, roughly 0.25 micron in diameter, composed of about 96% 
silica and having a light carbonaceous layer on their surface. Although 
the mechanism by which silica fume operates in the compositions of the 
invention is not entirely understood, its addition brings about a number 
of advantages, such as lowering the viscosity of the composition for a 
given solids content and reinforcing the gel network (without increasing 
viscosity) to give greater green strength. Without the presence of the 
silica fume, the hydrogel bonded aggregate structure appears more prone to 
cracking during drying operations. By reinforcing the gel structure, the 
silica fume reduces shrinkage as the article is dried. Generally, it has 
been found that the silica fume is effective at levels of from about 0.25 
to about 10 percent by weight of the total composition, preferably from 
about 1 to 4 percent by weight, and most preferably from about 1 to 2% by 
weight. 
As noted, gel strengthening agents other than silica fume can be employed, 
such as fly ash, manganese oxide fume, ferrosilicon fume and the like. 
Based upon experimentation to date, the chief characteristic required to 
be possessed by the gel strengthening agent is the small, spherical shape 
enabling it to react readily with the matrix binder and/or aggregate 
consttuents. tuents. 
As earlier noted, the moldable ceramic composition may advantageously 
further comprise refractory ceramic fibers, such as Kaowool.TM., 
Fiberfax.TM. and Fiberkal.TM. type aluminosilicate fibers, Saffil.TM. 
alumina fibers, silicon carbide whiskers and calcium silicate fibers, to 
give further rigidity to the fired structure. Typically, these fibers can 
be present in an amount up to as much as about 60 percent by weight of the 
composition, but most typically are employed in amounts from about 1 to 4% 
by weight. 
In the present invention, the components of the ceramic composition are 
selected to yield a particular setting time (e.g., by variation in 
aluminate/silicate ratio and/or solids content, and taking into account 
the temperature at which the composition will be molded or extruded), 
consistent with the anticipated duration of the foaming process in the 
mold or extrusion chamber. As noted earlier, a distinct advantage of the 
invention is that the setting time can be arranged to achieve a particular 
dimensionally stable degree of gelation at or very near the time when the 
gassing reaction ceases, thus insuring retention of the developed porosity 
in the eventually fired kiln furniture article. If gelation occurs too 
soon, the composition lacks the freedom to develop and accommodate the 
desired degree of porosity and/or may result in cracking of the set 
structure as gas continues to be evolved, while if gelation is delayed too 
long, the developed porosity will have a tendency to break down before the 
structure can be firmed up. While this latter problem might be curable by 
excess utilization of surfactant and/or metal powder, cure in this way may 
introduce into the article too substantial amounts of components making 
control more difficult and which may adversely affect final product 
characteristics. 
As noted earlier, the presence of silica fume in the composition results in 
substantial reduction of the viscosity of the composition, the measured 
reduction being greater at higher spindle speeds on the measuring device 
and also greater with increasing amount of silica fume. The green strength 
(as measured by the modulus of rupture or MOR) of the shapes generally 
increases with increasing silica fume content. Increase in the amount of 
surfactant or increase in available surface area of metal powder (increase 
in amount or also, e.g., by using either a flaked metal powder or smaller 
grain size) increases the number of pores per linear inch in the molded 
product. Increase in slurry temperature or other means to decrease set 
time results in an increase in density of the cast product, while a 
decrease in the available surface area of metal powder also increases the 
density. 
Following the removal of the molded porous ceramic shape from the mold or 
extrusion chamber, it is necessary to treat it to reduce or, ideally 
eliminate, alkali metal (e.g., sodium) therein prior to the firing process 
so as to avoid the formation in the fired kiln furniture article of glassy 
phases which would reduce the refractoriness or thermal shock stability of 
the kiln furniture. This may be accomplished by a number of techniques, 
but the most preferred is to contact the unfired porous shape with water 
to leach alkali metal compounds therefrom, and then to follow this with 
contact with a dilute aqueous solution of an ammonium salt such as 
ammonium chloride to effect substantially complete exchange of ammonium 
ion for any sodium ion remaining. A final rinse may then be employed to 
remove any residual chloride ion or other anion associated with the 
ammonium salt. 
Once the final rinse is completed, the parts are allowed to drain and dry. 
Drying is further enhanced by heating the component in a vented oven (or 
microwave oven) to about 400.degree.-600.degree. F. At this time, the part 
is transferred to a high temperature kiln and heated to the required 
firing temperature to allow the formation of a more homogeneous 
aluminosilicate ceramic bond in the component microstructure with an 
associated increase in strength. 
Thus, following removal of soluble alkali, the molded article is dried to 
remove water therefrom and is then fired in any suitable furnace at the 
temperatures required (e.g., 2200.degree. F. to 2600.degree. F.) to form 
the shaped porous ceramic kiln furniture article, a generally monolithic 
structure having porosity of th open-celled or closed-celled type. 
Depending upon the components of the composition and the processing 
conditions, sintered ceramic refractory kiln furniture articles can be 
prepared having a broad range of porosity. 
A wide range of refractory foam compositions can be achieved using the 
basic procedures outlined above depending on the specific requirements of 
the final kiln furniture product. For example, if thermal shock resistance 
is of paramount importance, refractory compositions that result in low 
thermal expansion can be incorporated such as those containing lithium 
aluminosilicate, cordierite (a magnesium aluminosilicate) and/or aluminum 
titanate. In addition, if strength and toughness are more important, then 
such materials as mullite, zirconia-toughened ceramics and ceramic 
composites may be incorporated. If high thermal conductivity is important, 
then the use of silicon carbide or silicon nitride is recommended. If high 
refractoriness is important, pure alumina can be used. If long term 
durability is required in both thermal and mechanical shock conditions, 
then low expansion, strong and tough type systems will be utilized. 
As noted earlier, the invention provides a porous ceramic kiln furniture 
element which, on one or more of its surfaces, is provided with thin 
porous ceramic membrane layer which serves as a smooth skin so as to 
provide a suitable support surface for supporting ceramic ware during 
firing and/or to provide a surface permitting the kiln furniture to be 
arranged suitably in the kiln. Pores in this skin. can be of open-cell or 
closed-cell type and can be in the form of sphcrcs, cylindrical channels 
or the like, and have an avcrage size (diameter) smaller than that of the 
pores within the kiln furniture article and at untreated surfaces. 
A wide variety of means can be used for providing this membrane on the 
porous, refractory aluminosilicate-based ceramic kiln furniture element 
according to the invention. 
In one such method, the porous ceramic shape, after formation but prior to 
firing, is treated by applying to one or more surfaces or areas thereof a 
ceramic paste or slurry containing a fugitive constituent capable of 
leaving a small pore when removed during the drying or firing operation. 
The fugitive constituent can be a sublimable compound or a burnable (e.g., 
carbonaceous) compound, utilized in a size and an amount which will result 
in pores having an average diameter smaller than that of the pores which 
will be present in the body portion or at untreated surfaces. During the 
firing operation, the ceramic paste or slurry becomes integrally 
associated with (fused to) the porous body portion. 
In another method, surfaces of the mold corresponding to the areas on the 
part where a smooth surface is desired are treated by application thereto 
(generally onto the mold release agents already present) of a mixture of 
ceramic powder and fugitive constituent. The composition is then poured or 
injected into the mold and, after setting and removal from the mold, will 
have associated with it at the areas corresponding to the pre-treated mold 
surfaces, a thin skin of ceramic material which is rendered porous during 
the firing step. In this embodiment, it is also possible to eliminate use 
of fugitive constituents by choosing for the ceramic powder ingredients 
which are more refractory than those of the underlying body portion, such 
that during firing, the greater refractoriness of these grains prohibits 
sintering thereby leaving a partially-sintered, i.e., porous, membrane 
layer on the preselected areas of the body portion. 
Among the preferred methods according to the invention involves the 
application of a ceramic paper (either woven, air-laid, or the like) atop 
the release agent on the appropriate mold surfaces prior to casting or 
injection molding the ceramic composition. In this manner, the 
composition, during foaming, expands into the ceramic paper, thereby 
laminating or bonding the systems together. On firing, there is developed 
a porous body portion having on one or more of its surfaces a thin porous 
ceramic membrane layer by reason of the now integrally-bonded ceramic 
paper whose pores are on the average smaller than those of the underlying 
body portion. 
In the most preferred methods, formation of a porous ceramic membrane layer 
is accomplished integral with the formation of the underlying porous body. 
In situ processing in this manner offers significant advantage in the 
economics of manufacture of the final ceramic kiln furniture article. 
According to one of these preferred methods, the release agent used in the 
mold, at the appropriate areas, consists of or contains a defoaming 
surfactant (i.e., a foam suppressor). During the internal development of 
porosity in the composition in the mold by virtue of gas-generating 
reactions therein, the defoaming agent acts to sufficiently suppress the 
reaction to keep the pores at these surfaces controllably small, i.e., 
smaller than those within the body portion and at surfaces not in contact 
with the foam suppressor. Since the surfactant is per se a release agent 
or is associated with a release agent, no problems are encountered in 
demolding the part. Commonly used surfactants for the defoaming of 
detergents, paints, varnishes and the like are eminently suitable for this 
purpose. 
According to another such preferred method, there is used, as the release 
agent per se or along with a release agent, a foam suppressing agent 
consisting of an organic compound having an unhindered hydroxyl group 
(i.e., an OH-- "tail"), such as common alcohols, polyethylene glycol, 
polyvinyl alcohol, and the like. By provision of such agents on mold 
surfaces corresponding to those areas of the body portion where the porous 
ceramic membrane layer is desired, the hydroxyl group apparently absorbs 
the outgassing hydrogen molecules at these surfaces, thereby restricting 
their growth. A porous ceramic membrane is attained by virtue of the 
underlying foaming reaction and the fact that hydrogen gas bubbles at the 
desired surfaces are kept small. 
In another method applicable to this aluminosilicate system, mold surfaces 
corresponding to those where a smooth porous ceramic membrane is desired 
to be formed are provided with a gel accelerating agent, preferably along 
with a release agent, and most preferably along with a release agent 
consisting of or containing an OH-tail as above described. The gel 
accelerating agent serves to locally set the aluminosilicate hydrogel 
prior to reaction between the particulate metal and alkali compounds in 
the composition. After removal from the mold of the set ceramic structure, 
the structure is treated to remove therefrom water and alkali metals 
(e.g., by leaching), such that at the surfaces where little or no porosity 
was developed as a result of the accelerated setting of the gel, small 
pores are developed by means of the water and alkali removal. 
Additional methods to achieve localized rapid gelation of the 
aluminosilicate system at surfaces where a porous ceramic membrane is 
desired include incorporation of water along with the release agent at the 
desired mold surfaces, the water being in an amount such that the 
combined, but not yet set, silicate and aluminate mixture absorbs a 
sufficient portion of this water to locally dilute the original amounts of 
soluble silicate and soluble aluminate, thereby locally reducing the gel 
time at these surfaces as compared to that occurring throughout the 
remainder of the composition. In another method, it can be arranged that 
water is locally removed from surfaces where a porous ceramic membrane is 
desired so as to bring about more rapid gelation of the aluminosilicate 
system at those areas (by virtue of increased solids concentration). This 
can be achieved, for example, by treating the corresponding mold surfaces 
with a hydroscopic release agent (or a release agent composition 
containing a hydroscopic agent) or by arranging a layer of dry paper at 
the required mold surface or by localized heating of the required mold 
surface. 
Another method applicable to the aluminosilicate hydrogel system is to 
bring about a change in pH on the surface where the porous ceramic 
membrane is required. For example, incorporation of an acidic component in 
the release agent such as acetic acid or dilute hydrochloric acid will 
locally accelerate the gelation prior to the onset of foaming. 
The invention is further described with reference to the following 
examples. 
EXAMPLE 1 
Two slurries were prepared, one containing sodium silicate and the other 
sodium aluminate. The slurries were prepared to a specific gravity of 2.1 
g/cc at a viscosity of 25,000 cps at 70.degree. F. 
______________________________________ 
Sodium Silicate Slurry 
sodium silicate grade 50 (44.1% solids) 
27.2% 
additional process water 5.4% 
Dow surfactant 190 0.6% 
silica fume (1/4 micron) 1.6% 
chopped fibers (1/8 and down) 
2.0% 
fused cordierite (-200 mesh) 
30.2% 
calcined kyanite (-200 mesh) 
32.7% 
powdered aluminum metal (6-9 micron) 
0.3% 
Sodium Aluminate Slurry 
sodium meta-aluminate solution (46% solids) 
25.9% 
additional water 5.7% 
Dow surfactant 190 0.6% 
silica fume (1/4 micron) 1.5% 
chopped fibers (1/8 and down) 
1.9% 
fused cordierite (-200 mesh) 
33.9% 
calcined kyanite (-200 mesh) 
31.0% 
______________________________________ 
Equal weights (360 g) of the slurries were combined and cast into a mold 
cavity having an 840 cc capacity (since the slurries had a specific 
gravity of 2.1 g/cc, only 41% of the mold cavity was filled). The mold was 
in the form suitable for preparing a 10-inch diameter plate, 5/8-inches 
thick. One side of the mold surface was coated with a release agent 
containing 17.5% polyethylene glycol 3350, 12.5% polyvinyl alcohol 
solution, 36.5% glycerine and 33.5% water. Approximately 30 seconds after 
the ceramic composition was cast into the mold, the mix began to foam to 
an open-cell porous structure having a wet density of 0.86 g/cc. Foaming 
stopped when the sodium aluminosilicate hydrogel binder phase set 
(approximately 3 to 4 minutes), freezing the expanded structure in place. 
Adjacent the release agent, the ceramic composition gelled more rapidly, 
thereby preventing growth of any hydrogen gas bubbles formed near this 
surface. After 8 to 10 minutes in the mold, the hydrogel developed 
sufficient strength to be demolded, and the supportable cast shape 
displayed on excellent smooth skin where it was in contact with the 
release agent composition. 
At this point the part contained 4.6% sodium oxide and 20.1% water at the 
above mentioned 0.86 g/cc density. In order to increase the 
refractoriness, the sodium oxide was then removed. This was accomplished 
by rinsing the part with 10 liters of purified water (deionized water with 
a 50,000 ohm resistance or better). This rinse reduced the sodium oxide 
content to approximately 2%, the stoichiometric amount. To remove the 
remaining sodium, the part was then subjected to 30-40 liters of a 1% 
ammonium chloride solution whereby all of the NH.sub.4.sup.+ ions 
replaced the Na+ ions. An additional 5 liter water rinse was then 
performed to remove excess Cl.sup.- ions after which the part was removed 
and allowed to drain and dry. 
After the initial draining and air drying period, the part was heated to 
600.degree. F. in 6 hours. The warm part was removed from the oven and 
placed directly in a kiln. The part was then slowly heated to the required 
firing temperature of 2425.degree. F. in 10-12 hours. Once at temperature, 
the part was held for 2 hours to complete the sintering operation before 
being allowed to furnace cool. 
The ceramic article made in this manner had a predominant microstructure of 
cordierite/mullite, an apparent refractoriness of about 2500.degree. F., a 
coefficient of thermal expansion of about 1.5.times.10(-6) at 700.degree. 
C. and 3.2.times.10(-6) at 1000.degree. C., and a room temperature modulus 
of rupture of about 400-450 psi. After exposure to 100 cycles of room 
temperature to 1250.degree. C., no significant loss of strength was 
recorded. The apparent refractoriness of the artile was about 2500.degree. 
F., and its density was about 0.6 g/cc. 
The article so made is excellently suited for use as kiln furniture by 
reason of its low mass, porosity, refractoriness, resistance to thermal 
shock, and smooth-skinned surface. 
EXAMPLE 2 
Utilizing the composition and process set forth in Example 1, but using 
instead a release agent consisting of 50% water and 50% glycerine, a 
porous refractory ceramic kiln furniture plate of low density was produced 
having a smooth skin on one surface thereof by reason of rapid gelation of 
the aluminosilicate hydrogel in contact with the release agent prior to 
development of any large bubbles. 
EXAMPLE 3 
Utilizing the composition and process set forth in Example 1, but coating 
one of the mold surfaces with a thin layer of woven mullite fibered paper 
(instead of using the release agent composition of Example 1), the part 
removed from the mold was found to have the woven paper significantly 
attached to its surface such that no separation occurred during subsequent 
sodium removal processing. Upon firing, there was produced a low density 
porous ceramic refractory kiln furniture article having a smooth skin on 
one of its surfaces. 
EXAMPLE 4 
The same composition and process set forth in Example 1 was employed with 
the exception that a silicone release agent modified with a silicone 
defoaming surfactant was sprayed on one of the mold surfaces, and that 
mold surface was locally heated to about 140.degree.-150.degree. F., while 
the other mold surface was kept at room temperature. As a consequence of 
accelerated gelation of the hydrogel in contact with the heated surface 
insufficient time was available for the foam cells to grow to any 
appreciable size before setting occurred. The result was a smooth surface 
on the kiln furniture article after firing. 
Having described the invention with reference to particular compositions, 
processes, examples and embodiments, it is to be understood that these 
particulars are presented for purposes of illustration and description, 
and are not otherwise intended as strict limitations upon the scope of the 
fundamental invention as defined in the appended claims.