At Dodero Studio Ceramics, we use one clear base glaze over a white body with the addition of Mason and Ferro glaze stains of 5% to 10%. When using these types of Raku glazes there are many things to consider for a positive outcome; the following may be helpful on some of these points.
-Bisque temperature of ware; if ware is bisqued too high, glaze absorption can be uneven or it may be hard to get a thick enough coat for ample coverage and proper color density.
-Thickness of glaze application; if application is too thin, the glaze will have a washed out look. If the glaze is too thick you can get pin holing, mottling, or the glaze may run off the pot.
-Viscosity of glaze; in order to get a good glaze coat when dipping, a reading of around 42-45 on a hydrometer should be about right. Glazes with a high amount of Gerstley Borate should be weight tested because of a tendency of overfluctuation caused by Gerstley B. 500ml of glaze should weigh about 720 grams = 1.44 specific gravity. At proper specific gravity, hold the pot under for about 20-25 seconds. This should give you a coat of about 2/100 of an inch. If the pot is bisqued to proper temperature (around 1000c in oxidation depending on your clay body) when spraying (for larger pieces that are more difficult to dip) a reading around 40 is good.
-Technique of glaze application, spray, dip; The technique of application should match the piece and the amount of glaze available, if you have a small amount of glaze, spraying or brushing may be your only option.
-Shape of ware to be glazed. Some shapes that have wide shoulders with a small foot can curtain if the glaze is too thick or over flocculated.
Two possible solutions are: when dipping when the pot is emerging from glaze at end of dip, remove very slowly form the widest spot down, letting glaze sheet evenly to avoid curtaining. Spraying, though time consuming, may be the only way to get an even coat.
Dipping will take a container of glaze that is large enough to fit the piece with room to spare. I do not recommend pouring for even coverage with this type of glaze.