Glaze is, by definition, “a vitreous substance fused on to the surface of pottery to form an impervious decorative coating.” Mixing pottery glazes takes knowledge: Knowledge of the components themselves, and knowledge about how best to put them together.
Silica: The material that forms the glass Alumina: A refractory and stiffener that allows the glass to stick to vertical and even overhanging surfaces while it is molten; it is what makes glass into the glaze. Fluxes: Makes the silica melt at a lower temperature than it would otherwise. Colorants: Adds color to the glaze Modifiers: Changes the glaze in some other manner, such as opacifiers, those that add opalescence, or those that encourage crystal growth.
The reality of glaze ingredients is not as easy as looking around a chemistry lab for pure chemicals. The most basic glaze ingredients come in combination forms; for example, alumina and silica are found in every type of clay as well as rocks such as feldspars. On the other hand, many colorants and modifiers are sold to potters by their chemical name or formula; for example, red iron oxide (Fe203) is one of the most commonly used ceramic colorants. In addition, you should always wear lung protection. A dust mask is simply not enough. Other supplies include:
Water: Glazes should be mixed using distilled water only.Three or more buckets: Make sure they have air-tight lids.Scales: All measurements are by weight. You will need an accurate scale that can handle the amount of weight.Skin Protection: Use latex gloves.Sieve: A large sieve or screen.Mixer: More on this below.
Potters have two choices of mixer drill attachments. The first are the ones made especially for mixing glazes. These are expensive but tend to be designed to both reduce air being added to the liquid glaze and to handle the density of the glaze. The second choice is paint mixers, also called paint stirrer attachments. These are cheaper, may be easier to find, and do the job. They will, however, be more likely to add some air into the liquid glaze, which affects application of the glaze to the pottery. Always measure your ingredient in a scoop or container, not loose on the scale. Make certain to tare the scale, whether physically or digitally, so the weight of the container is subtracted. Be sure to have latex gloves on before you begin handling raw glaze ingredients. Some of them can enter the body through the skin or through cuts. With respirator and gloves on, sieve all measured dry ingredients together, similar to sifting flour and baking powder together. It is easiest to measure and dump ingredients into one (dry) bucket, then sieve them into a second dry bucket. For a dry batch weight equaling 20 pounds, begin with about 2 gallons of distilled water in a 5-gallon bucket. The final consistency for most dipping glazes will be like that of heavy cream. If you want more precise control, you can use a hydrometer and determine your own preferred specific gravity for your glazes. Keep the mixer attachment, whisk, or hand fully submerged while it is rotating. If using your hand, break up any lumps you feel. With the whisk and mixer attachment, move the blades so as to mix all areas. Let the glaze rest for about 15 minutes before using it. This allows any air to rise to the top and escape, as well as gives all the particles a chance to become thoroughly wet. Remember, glazes settle, and some settle more rapidly than others. If it has been longer than a few minutes since you have used the glaze, stir it up again with a long stir stick or the mixer attachment. If you use up your batches quickly, you should not have too much trouble with the glaze ingredients settling into concrete-like sediment on the bottom of the bucket. If you are storing glaze for longer than a few days, shake or stir them up thoroughly every few days. If, for some reason, you are decanting the glaze into several smaller containers (such as sharing it with potter friends), be sure that all particles are fully mixed and in suspension. Label all the containers and note any hazardous material.
