Adjusting for Molten Viscosity: When using a glaze always calculate the glaze to a 100% batch with coloring oxides, stains, or suspension agents listed after the 100%. When adjusting a glaze by either adding or subtracting move materials by 5 part increments. When a modification is successful the revised glaze can easily be recalculated to the 100% batch weight.

Using the Raku Base Clear Gloss Crackle glaze several adjustments can change its viscosity, opacity and color. By adding 5, 10 or 20 more parts EPK to the base it will result in a stiffer glaze and less likely to run. Essentially by adding more EPK which contains alumina and silica both refractory oxides you are making a higher temperature glaze while still firing to the original temperature. However, as the amount of EPK increases the glaze becomes more opaque and develops a drier surface texture. Conversely, deleting 5, 10 or 20 parts of EPK from the base will make the glaze more fluid. Additions or deletions of EPK in the glaze can control molten viscosity and light transmission (transparent to opaque).

Another method to adjust the flow of the glaze is to work with the flux part of the formula (Ferro frit #3110).  Using the original glaze formula with additions of Ferro frit # 3110 can cause the glaze to become increasingly fluid while decreasing the frit can cause the glaze to become less fluid when molten.

Adjusting for Glaze Opacity

Another method for creating glaze opacity other than adding EPK which can affecting surface texture is the addition of zirconium silicates such as Zircopax plus. Tin oxide additions will also result in opacity with a softer white as compared to zirconium silicates. A 1% addition of either will yield a slight opacity while 5% to 20% will produce complete increasing opacity in the glaze.

Adjusting for Glaze Color

Stains or metallic coloring oxides can be used in the original glaze formula or in addition to opacity producing materials. Generally, a 1/8th % addition of stain or oxide will cause a tint, while 5% will cause a half intensity of color and 10% to 12% will yield the full intensity of color to the fired glaze.

Adjusting Glaze fit

The spacing of the crackle lines (crazing a fine network of lines in the fired glaze surface due to the glaze being under tension) will depend on the fit of the clay body and glaze upon cooling. Different frits have various rates of contraction when cooling which can enlarge or expand the craze lines in the glaze. Additionally, other glaze materials in this glaze such as EPK can influence crazing by introducing silica and alumina. The craze line pattern is also affected by the rate of contraction in the clay body being used for the Raku process.

To stop crazing in this glaze several strategies can be used by altering the glaze or clay body. Another frit with a compatible rate of cooling to the clay body will bring the glaze under a slight compression and possibly stop crazing. A different clay body can also set up a compatible glaze/clay fit. Metallic coloring oxides, their carbonate forms, stains and zirconium silicates can also affect glaze fit. In some instances craze lines are in the glaze but the glaze colorant masks their appearance. Glaze fit can also be improved by recalculating the glaze through glaze calculation software. Fundamentally, the Raku process with a highly absorbent clay body needed to withstand the thermal shock in the firing process will induce crazing in most glazes. Lead or lead frits promote glaze fit but are not recommended for health issues and possible lead leaching in the fired ware.