|Monthly Tech-Tip |
A densification process occurring within a ceramic kiln. With increasing temperatures particles pack tighter and tighter together, bonding more and more into a stronger and stronger matrix.
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.
The sintering process begins in a clay when it has been fired high enough so that it no longer will slake or break down when exposed to water. Bisque fired ware has advanced to a higher state of sintering. Refractories can often be sintered to considerable strength because the high temperatures maximize particle packing and surface interaction. Sintered alumina or zircon bodies, although not vitrified, can have a 'ring' like that of a fine porcelain. Some sintered bodies achieve even greater particle bonding by the deposition and buildup of material that has become gaseous in the kiln atmosphere. And interesting advantage of sintering over vitrification is that the hot strength of a body at temperature A will be greater if it is first fired at a temperature higher than A. If it is fired again even hotter (assuming of course that no melting is occurring), it will have even more strength at temperature A.
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.
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.
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).
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.
The particles in low temperature bodies are not glass-bonded, they only have sinter bonds. Broken edges will only be sharp if there is a glaze. They tend to break off in pieces rather than shatter.
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.
Three mugs. Dry. Bisque fired. Glaze fired. Notice the shrinkage at each stage (these were the same size in the dry state).
The powder was simply put into it and fired. Sintering is just beginning.
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.
Both of these bodies have a range of particle sizes (from 42 mesh and finer). The glaze on the cone 04 section (close-up on the left) is very well melted, but its interfacial zone with the body is narrow, it is basically just "stuck on" the surface (so it must be tuned to match the thermal expansion of the body to prevent crazing/shivering). The body is not developing any clearly visible glassy phases, meaning that any strength it has is purely a product of sintering. On the right, the interfacial zone of glaze-with-stoneware-body is wider. The body has incipient melting (tiny feldspar particles are fusing and beginning to dissolve into the surfaces of surrounding ones). The color has changed from red to brown.
|Temperatures||Sintering and densification (850C+-)|
The use of some traditional firing techniques is still popular among modern potters and sculptors (who are accustomed electric and gas kilns, often with computer controllers).
Ceramics, by their brittle nature, have high compressive strength. But in functional ceramics we are more concerned about the tensile strength as this relates better to serviceability.
Man-made ceramic surfaces are among the most abrasion resistant materials known. Products made to abrade others are also made from bonded ceramic grains.
Ceramic materials are among the hardest and most heat resistant materials known. Ceramics spans the spectrum from ancient terra cotta to modern hi-tech materials.
Ceramic stains are manufactured powders. They are used as an alternative to employing metal oxide powders and have many advantages.
In the ceramic industry, refractory materials are those that can withstand a high temperature without deforming or melting. Refractories are used to build and furnish kilns.
|Oxides||Al2O3 - Aluminum Oxide, Alumina|
Sintering on Wikipedia
Formulating a Porcelain
The principles behind formulating a porcelain are quite simple. You just need to know the purpose of each material, a starting recipe and a testing regimen.