GLAZES - USES & APPLICATIONS

Glazes can be thought of as a glass coating, either clear transparent or, in a rainbow of colors. They may also be glossy or matte, translucent or opaque, smooth or textured. The effects are limitless depending on formulation, application and combinations. Never be afraid to try new combinations or techniques but always test first.

Glazes are formulated as a mixture of ground, powdered ingredients which, by their nature, do not dissolve but are merely suspended in water. Glazes do not ‘go bad’ with age but, because different ingredients tend to come out of suspension at different rates, it is critical that the batch or bottle be mixed thoroughly before each application. This may be done with a spatula, whisk, an electric mixer or simply by shaking a bottled glaze, but one must be certain there is no glaze left caked on the bottom.

Good glazing begins with properly fired and clean bisque. Single firing (glaze applied to greenware) is not recommended with today’s glazes. Properly fired bisque has been fired to a temperature high enough that organics and water have been driven off and certain other reactions have occurred but not so high that the porosity necessary to ‘hold’ the glaze on (both prior to and through the firing) has been eliminated. Technically, this would be different for different clays but Cone 010 to Cone 02 is a reasonable range. Generally, low fire (Cone 05-06) glazes on low fire clays should be bisque fired 2 to 3 cones hotter (Cone 04 to 03) than the recommended glaze firing temperature. Wetting overly porous bisque will reduce the porosity and help in getting an even glaze application. If bisque is not porous enough due to over firing or if additional glaze needs to be added to an already glazed and fired piece, heating the piece in the oven or on top of the kiln before applying the glaze may help.

Glazes are not "sticky" but adhere to bisque before firing by penetrating the porous clay and after firing this bonding is increased by a degree of interaction of the glaze melt with the surface of the clay. Thus, any dirt, dust or oil on the bisque that even slightly prevents the glaze from soaking into the clay can cause the glaze to "lift" or "crawl" in firing, leaving unglazed spots or pin holes due to excessive gas release. Prepare bisque by wiping it with a clean wetted and squeezed sponge, chamois or other non-shedding material to remove dust. Make certain your hands are clean when handling bisque. Even body oils and hand lotions can cause glaze problems.

DRY GLAZE MIXING

Before we get into dry mixing, please note to ALWAYS WEAR A RESPIRATOR OR N-95 OSHA APPROVED DUST MASK.

Dry glazes should be mixed with water in a general ratio of 1 lb (16 oz) dry glaze to approximately 9-12 oz of water. Clear glazes usually need to be slightly thinner than colored glazes to keep them from being too cloudy.

 All glazes vary slightly in how much they hydrate and accept water, so start by measuring out 10 ounces of water for every pound of dry glaze you have in a bucket or appropriate size container.

Add the DRY GLAZE TO THE WATER while vigorously stirring the water until it is fully incorporated. A spatula, whisk, immersion blender, or drill mixer will work. 

 Glazes thickness and viscosity can be measured and eyeballed by dipping a finger and having full coverage, hiding your skin tone through the glaze. Glaze consistency should be that of heavy cream. No Yogurt. It should move and be able to pour easily.

 It is best to pass your glaze through a screen into a second container to be sure you mixed in and dispersed the dry material evenly with no chunks. Most glazes will pass completely through a 60 mesh, 80 mesh, or even a 100 mesh screen.

 Let your freshly mixed glaze sit for a day so the materials completely hydrate together. You can add more water if necessary to achieve a proper thickness. Add small increments of water mixing between additions if too thick. If too thin, a glaze can be left to sit a few days or until it starts separating, and clear water can be removed/skimmed from the top of the glaze. 

Always fire a test piece or test tile by dipping the test in the glaze after a good mixing, dipping the test piece in the glaze for a 2 or 3 count. Do this before you glaze your final products. The test piece will tell you if you need to add more water if glaze is too thick, or remove water if glaze is too thin.

 If the test comes out well, you are ready for glazing your pieces!

BRUSHING vs. DIPPING

BRUSHING
Glazes meant to be brushed on like those sold in pint jars normally have additional ingredients to enhance their flow off the brush and the evenness of their application. Some dipping glazes need no alteration for brushing, but if they do, the following approximate additions to one pound of dry glaze will help (test first with lower additions):

Mix dry glaze like above with 8 to 9 ounces of water to one pound dry glaze (similar to basic dipping glaze formula but with less water).
Add 4.5 to 7.5 grams bentonite
Add 2.3 to 3.4 grams CMC gum (first dissolved in warm water and aged overnight)
Note: 2 teaspoons equals 5-6 grams

BRUSHING APPLICATION

When brushing a glaze, use as large a brush as reasonable with a smaller brush for hard to reach places. Use a full brush of glaze at all times. Dip the brush in the glaze and shake it gently, just sufficiently to prevent dripping. Do not scrape the brush on the top of the bottle or there will not be sufficient glaze on the brush to flow freely. Hold the brush lightly and flow the glaze off the brush in a continuous line and never "pat" glaze on randomly (except for special effects). Smooth the glaze by brushing back and forth very lightly. Begin the next stroke where the previous stroke ended and then lightly brush back over the area where the two strokes joined, which will adequately smooth out the "joint" between the two strokes. Apply subsequent coats in the same manner but in a different direction (for example, first coat horizontally, second vertically, and third, diagonally). Wait for the previous coat to dry thoroughly before applying another coat (A matter of a few minutes, After the first coat loses its wet "sheen").