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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 (up to 1.8) are employed, the low water content speeds drying (thixotropy is not generally a concern, low water content is the main issue). The spraying process is most suitable for the single-fire ceramics.
For best results:
-Glazes need 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.
-Gravity feed guns are the most trouble-free.
-Use a sticky kaolin as the clay portion (e.g. Grolleg).
-For lower specific gravity glazes, adjust the slurry so that it is thixotropic, that is, so it stays in suspension in the sprayer and is less likely to create runs.
-Use glazes high in frit and low in troublesome raw materials (ones that affect slurry viscosity or thixotropy, are soluble and crystallize in the slurry, 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 or find an efficient way to screen them to 100 mesh.
-Make sure the glaze feed is well screened so it has no particulate matter.
-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, spall or crack.
-Make sure your spray operator is skilled and has 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 (and crawling during firing).
-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.
-Incorporate CMC gum, if it can be tolerated, to improve bonding to bisque and dry harder.
Again, the rheology of your slurry is very important for quality, efficiency and avoiding production issues. Experimenting is needed to determine the best specific gravity. Document the slurry preparation, measurement and maintenance procedures well.
For good spraying technique watch the video provided below. It was graciously given to us from http://shower-shelf.com. They also document their vibratory glaze sieve on youtube (links below for it and where to get the vibration motor).
Notice he starts from the bottom and works his way upward.
Spray glazed large bowl with two glazes meeting-at-the-rim
The inside glaze is G2571A, outside is the bamboo version of the same recipe (with added Zircopax and rutile). Clay: Plainsman H443. Firing: Cone 10R. No wax was needed for application yet the two colors meet at the rim in a straight line (the outside was sprayed last). We used a gravity-feed sprayer, 100ml of glaze was applied on both the inside and outside.
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