The term "sintered" refers to the particle-to-partice bonding and packing that occurs within a ceramic matrix as temperature increases. Sintering is a process as well as a state. Sintered bodies are not vitrified; the process occurs without any glass development (melting) to glue particles together. Rather, adjacent particles bond or fuse together at points-of-contact by the migration of molecular species across the boundaries. This exchange accelerates with increasing temperature as surfaces become more and more chemically active. In addition, increasing temperature makes particle surfaces more slippery, they are freer to rearrange in an ever increasing density. Finer particle sizes, likewise, will increase surface area and increase strength even further.
How much does the size of a piece change when it is bisque fired? Glaze fired?
Three mugs. Dry. Bisque fired. Glaze fired. Notice the shrinkage at each stage (these were the same size in the dry state).
National Standard Bentonite fired to 1650F in a small crucible
The powder was simply put into it and fired. Sintering is just beginning.
What is sintering?
Bentonite fired to 1950F in a small crucible. It is sintering, the particles are bonding even though there is no glass development. The powdered mass is behaving as a unit, the cohesive forces holding it together are enough to shrink the entire mass away from the walls of the container. This sintering process continues slowly, beginning around 1650. Most raw bentonites, this is National Standard 200 mesh, have a fairly low melting point, this will begin to fuse soon.
The difference between vitrified and sintered
The top fired bar is a translucent porcelain (made from kaolin, silica and feldspar). It has zero porosity and is very hard and strong at room temperature (because fibrous mullite crystals have developed around the quartz and kaolinite grains and feldspar silicate glass has flowed within to cement the matrix together securely). That is what vitrified means. But it has a high fired shrinkage, poor thermal shock resistance and little stability at above red-heat temperatures. The bar below is zirconium silicate plus 3% binder (VeeGum), all that cements it together is sinter-bonds between closely packed particles (there is no glass development). Yet it is surprisingly strong, it cannot be scratched with metal. It has low fired shrinkage, low thermal expansion and maintains its strength and hardness at very high temperatures.
Can you make things from zircopax? Yes.
Only 3% Veegum will plasticize Zircopax (zirconium silicate) enough that you can form anything you want. It is even more responsive to plasticizers than calcined alumina is and it dries very dense and shrinkage is quite low. Zircon is very refractory (has a very high melting temperature) and has low thermal expansion, so it is useful for making many things (the low thermal expansion however does not necessarily mean it can withstand thermal shock well). Of course you will have to have a kiln capable of much higher temperatures than are typical for pottery or porcelain to sinter it well.
Bisque temperature can be lower than you think
These bowls are made from a talc:ball clay mix, they are used for calcining Alberta and Ravenscrag Slips (each holds about one pound of powder). The one on the right was bisque fired to cone 04 (about 1950F). The one on the left was fired to only 1000F (540C, barely red heat), yet it is sintered and is impervious to water (strong enough to use for our calcining operations). That means that there is potential, in many production situations, to bisque a lot lower (and save energy). Primitive cultures made all their ware a very low temperatures. Tin foil melts at 660C (1220F) yet can be used on campfires for cooking (so the temperatures of primitive wares would have been low indeed).
My first zircopax kiln shelf passed with flying colors
It is 5 mm thick (compared to the 17mm of the cordierite one). It weighs 650 grams (vs. 1700 grams). It will perform at any temperature that any kiln that I have will generate and far in excess of that. It is made from a plastic body having the recipe 80% Zircopax Plus, 16.5% 60-80 Molochite grog and 3.5% Veegum T. The body is plastic and easy to roll and had 4.2% drying shrinkage at 15.3% water. The shelf warped slightly during drying, so care is needed. First-firing at cone 4 yielded a firing shrinkage of 1%). Notice that cone on the shelf: It is not stuck so no kiln wash is needed! Zircopax is super refractory! It is held together by sinter bonding, so the higher the temperature you can fire to the stronger it will be.
The silica pants of this cone 10R mug have fallen down!
This stoneware mug was glazed inside and halfway down the outside with pure silica. At some point during heatup the outside layer, not shrinking like the piece, simply fell down. And was sintered enough to hang together and remain intact through the rest of the firing (on the inside, the shrinking forced the silica to flake off into a pile at the bottom). Cone 10 has sintered the silica enough that it will not slake in water but it is fragile and soft and must be handled carefully.
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