While small changes in water content can be employed to fine-tune viscosity and thixotropy, bigger adjustments need to be handled by additives to flocculate or deflocculate the slurry. However, if a glaze is working may never be necessary to use additives if you are using quality materials, consistent water, and a recipe that has adequate clay. If your water varies seasonally, then use distilled or RO water. If your materials are slightly soluble or contain soluble materials that disrupt glaze flow properties, then use cleaner materials. Ware Bisque ware should be low in porosity.
Variable porosity means variable thicknesses in the overlying glaze.
Porous bisque ware demands that glaze slurries be thin and runny or the application will be too thick. If you are used to bisque firing from cone 010 to 06, go to 04 or higher if you can. While thixotropic behavior can be achieved by using glaze additives, most people lack the experience and knowledge to use them properly. It is thus desirable to avoid additives if possible and try to select a kaolin or ball clay that contributes thixotropic properties. Remembering to compensate for their different chemistries by calculation if necessary.