GLAZES - USES & APPLICATIONS
Glazes can be thought of as a glass coating, either clear transparent or, with the addition of a metal in any of several forms, in a rainbow of colors. They may also be glossy or matte, translucent or opaque, smooth or even 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, 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 by opening the pores.
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 wetted and well squeezed sponge, chamois or other non-shedding material to remove dust. Make certain your hands are clean when handling bisque. Even body oils can cause problems.
APPLICATION - DIPPING, SPRAYING & BRUSHING
Glazes meant to be brushed on often 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 half the addition):
10.5 to 11.5 ounces of water(basic brushing glaze formula)
4.5 to 7.5 grams bentonite
2.3 to 3.4 grams CMC gum (first dissolved in warm water and aged overnight)
Note: 2 teaspoons equals 5-6 grams
Ideally, viscosity should be 2200 to 2500 on a hydrometer
Dipping glazes should be a thick cream consistency. An old "rule of finger" is that when a clean dry finger is dipped in a mixed dipping glaze, the glaze should coat the skin evenly and bead on the finger nail. The reality is that the glaze needs to be test fired on the properly bisque-fired clay body it will be used on. It should be of such consistency that a single dip, leaving the piece in the bucket several seconds, will provide the proper glaze thickness. A general mixing formula follows (use less water first or leave aside some dry glaze to allow adjustment):
Add to 1 pound dry glaze:
6.5 to 7.5 ounces of water
0.0 to 2.3 grams bentonite (if needed to maintain suspension)
Ideally, viscosity should be 900 to 1000 on a hydrometer.
Glazes to be sprayed also require testing but the following can be used as a guide for mixing one pound of any glaze for spraying (always use a NIOSH approved mask when spraying):
7.5 to 8.5 ounces of water
0.0 to 3.4 grams of bentonite
The viscosity should be 1500 to 1700 on a hydrometer
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").
An almost endless number of special effects can be achieved by modifying the above techniques but should be tested first. Things to try include the following, individually or in combination (test first and avoid more than the recommended number of total coats):
• Uneven application (usually less or watered down glazes unless on upper part of the piece where extra glaze won’t run off on to the shelf when fired).
• Sponging or dappling or spritzing the same glaze or another glaze on top of the coat (or second coat if brushed —on glaze)
• Overlapping a second adjoining glaze on the first color
• Strategically adding areas of underglaze over the glaze
• Using lower firing glazes (or underglazes) at higher temperatures either alone or over (or under) proper temperature glazes. Nearly all low fire (Cone 05-06) glazes and underglazes can be fired to Cone 5 or even higher but will lose some intensity (especially reds, pinks, and yellows). Be cautious though as the glazes will tend to run much more at the higher temperatures.