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In the production of smaller bisque fired ceramics it is almost always possible to dip-glaze ware. However, this is seldom an option for single-fire ware (especially if large). This is the case in the sanitary ware industry, for example. Spraying is the only option, and it is a very effective one if done properly. Typically opacified glossy glazes of high specific gravity are employed (the low water content speeds drying). The spraying process is one that could be adopted by many smaller operations that are interested in moving to a single fire process.
For best results:
-Your glazes should contain enough clay to suspend and harden them, but not so much that they dry too slowly or shrink too much (thereby cracking or flaking off). 10-20% should be OK.
-Use a sticky kaolin as the clay portion (e.g. Grolleg).
-Adjust the slurry so that it is thixotropic, that is, so it gels on contact and stays put (does not run downward). Slurries of higher specific gravity are much more difficult to make thixotropic and are more likely to run if spray too thickly. You need to decide what balance between low water content and gelling behavior is best for you.
-Use glazes high in frit and low in troublesome raw materials (ones that affect slurry viscosity or thixotropy, ones that are soluble and crystallize in the slurry or create a precipitate scum inside containers or ones that contain excessive particulates).
-Use the right frits. They need to source the needed fluxing oxides and complement the recipe to optimize clay percentages. For example, if a glaze needs more clay then the frits employed should have lower Al2O3 content so that can be be sourced from a kaolin instead (or vice versa).
-Ball mill your glazes.
-Spray ware when it is sill warm to encourage faster drying.
-Make sure ware is thick enough so that it will be able to absorb the amount of water needed to set the glaze and that it will not re-soften and then warp or split.
-Make sure your spray operator is skilled and he had good equipment. Spray an even layer. Start at the bottom and work upward. Be careful about over spraying already-dry areas, if done excessively this can rewet them and cause lifting during drying or crawling during firing.
-Make sure the glaze feed is well screened so it has no particulate matter.
-Spray the ware on a rotating turn-table having a knobby or spiked rubber or plastic mat on which to set the item.
-Use a good quality stainless steel agitated pressure pot sprayer (see link below).
-Apply the thinnest possible layer needed to get the desired fired effect.
Again, the rheology of your slurry is very important. The higher water content you can tolerate in the glaze the more you can gel it (increase the thixotropy) but the longer it will take to dry. Much experimenting is needed to determine the best specific gravity to use and you must document the slurry preparation, measurement and maintenance procedures well. If the slurry is not the right consistency it will cause all kinds of production issues.
A comparison of the weights of equal volumes of a given liquid and water. Water has a specific gravity of 1.0. A ceramic slurry with a specific gravity of 1.8 is thus 1.8 times heavier than water. The best way to measure specific gravity is to weigh a container and record its weight, then weigh the ...
Knowing about thixotropy enables you to mix non-gummed glazes that stay in suspension much better. But it is not only about staying suspended. While some glazes do not settle out they that have a slurry that behaves like a bucket of motor oil, silky smooth but they just drip and drip and drip. Thixo...
The term viscosity is used in ceramics most often to refer to the degree of fluidity of a slurry or suspension (the term 'shear' is often used when discussing viscosity, theoretically engineers understand viscosity in terms of layers particles or molecules that exhibit a friction that resists latera...
In ceramics, when we speak of deflocculation, we are almost always talking about making a casting slip. Glazes can also be deflocculated (to reduce water content and densify laydown).
Deflocculation is the process of making a clay slurry that would otherwise be very thick and gooey into a thin po...
The practice of applying glazes to dried ware and firing in one operation. Obviously this is going to save money on energy. But it introduces extra problems also. In general, the thicker and heavier the ware and the greater its dry strength the greater the chance that it can be glazed easily in the ...