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Education News
Clay | Glaze
| Firing | Toxicity
| Materials | Books
| People | Events
| Glossary
We often tend to put so much effort into adjusting our glazes to fine-tune fired properties that we tolerate poor application properties. Such glazes are not only frustrating to use, but they usually produce poor fired results.
When a slurry is right it should 'gel' and 'hang on'. You should be able to dip your finger in and pull it out with an even coverage and no drips. In fact, a thixotropic glaze will not even shed off ware covered with wax emulsion! And it will never settle out in the container! Yes, there is no reason to put up with a glaze that drips and drips and does not produce an even layer.
In industry, maintaining the 'rheology' (flow properties) of the glaze slurry through material, water, seasonal, and personnel changes is often the most difficult challenge a factory faces. Maintenance of the specific gravity thus becomes a reference point, 'an anchor' around which all other adjustments hinge. Thus, if your glaze is working well, measure its specific gravity now, and try to maintain this value in future. While small changes in water content can be employed to fine-tune viscosity and thixotropy, bigger adjustments need to be handled by additives to flocculate or deflocculate the slurry.
However, if a glaze is working may never be necessary to use additives if you are using quality materials, consistent water, and a recipe that has adequate clay. If your water varies seasonally, then use distilled or RO water. If your materials are slightly soluble or contain soluble materials that disrupt glaze flow properties, then use cleaner materials.
Bisque ware should be low in porosity. Variable porosity means variable thicknesses in the overlying glaze. Porous bisque ware demands that glaze slurries be thin and runny or the application will be too thick. If you are used to bisque firing from cone 010 to 06, go to 04 or higher if you can.
The glaze slurry must be thixotropic, it must "gel" so that the mechanism of its initial adherence to the ware is, to a considerable extent, a function of this property rather than absorption of water by porous bisque. While thixotropic behavior can be achieved by using glaze additives, most people lack the experience and knowledge to use them properly. It is thus desirable to avoid additives if possible and try to select a kaolin or ball clay that contributes thixotropic properties. EPK (kaolin) is a good example.
If your glaze does not contain adequate kaolin (15-20%) then use ceramic calculations to adjust it so that it does. It is possible to have 20% kaolin in cone 04 glazes if you use low alumina boron frits. At higher temperatures glazes have significantly more Al2O3 and SiO2 and so it is usually easy to achieve a 25% kaolin content. Experiment with the kaolins and ball clays available to you to find the best one (we like EPK). Remembering to compensate for their different chemistries by calculation if necessary.
As a last resort trying adding a flocculant such as calcium chloride, Epsom salts, or vinegar to a small amount of the slurry and check the results. Often only a few drops are needed so be careful not to add too much and end up with a useless jelly-like mass.
Misuse of glaze additives is very common because they are not nearly as well understood as other materials. Individual additives often defy easy classification imparting suspending, adhesive and flow properties. Thus picking the right one is a matter of discerning the need and using the additive that 'emphasizes' the needed slurry property and gives the fewest side effects (i.e. color change, slower drying rate, biodegradation, film formation). Many people use additives that actually worsen the application properties of their glazes when a recipe change would be much better (i.e. gum additions may give a slower drying 'drippy' glaze when a simple addition of bentonite, change of kaolins, or reformulation to increase kaolin would be more effective).
While gum does form a gel to suspend particles, it is usually more useful in making the slurry 'sticky', and acts as a temporary glue to cement otherwise loosely adhered particles; thus it is referred to as a 'binder', 'hardener', 'adhesive'. While clays such as ball clay and kaolin may suspend glaze particles, they usually do not impart significant gelatinous qualities. Starches usually act as hardeners and may thicken the slurry. Electrolytes change the pH of the suspension and affect the charge of particles changing slurry viscosity; a few drops can make a thick slurry very runny and thin, or make a thin one gel (thus they can be used to keep otherwise variable flow properties constant as well as impart a flocculated or deflocculated consistency). Veegum is a claylike plasticizer so it will impart similar properties. Cellulose ethers are used like gum and starch to harden and thicken, they are more consistent and easier to control. Other additives include wetting agents, foam control agents and sealers. People who know how to use these materials can do things with glaze that others might think impossible.
Our general advice is this: If your glaze is not suspending, hardening, or gelling properly then look at the recipe and reformulate it use more clay. If it still needs help then add bentonite (up to 3% in some cases). Bentonite suspends, hardens and helps the glaze gel when it stops moving. If the glaze still needs extra help, then use an additive from the ones listed above.
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