Glaze Batch Adjustment After a Test Tile Comes Out Wrong
A studio mixes a 10-liter bucket of a satin white glaze from a recipe scaled to 1000 g dry materials. The batch has been dipping too thin: test tiles show a 0.2 mm layer where the studio target is 0.4 mm. The coordinator adds 200 g of dry mix and 300 mL of water, then dips a new tile that reads 0.35 mm but shows pinholing and a slightly matte, underfired surface. The kiln is fired to cone 6, and the glaze recipe includes 10% frit and 5% bentonite. Another student needs the same glaze for a large platter and asks whether to dip longer or add more water.
1. Diagnose why the adjusted batch pinholes and reads matte despite a thicker layer, and decide the next corrective step.
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Adding 200 g dry mix to a 1000 g batch increases solids by 20%, which raises specific gravity and thickens the slurry, but the simultaneous 300 mL water addition dilutes it back and changes the water-to-solids ratio unevenly. The result is a heavier, less well-dispersed suspension: bentonite at 5% swells and can trap air, producing pinholes, while uneven mixing leaves frit-rich and frit-poor zones that fire matte in patches.
The matte surface at 0.35 mm is not an underfire problem if the kiln reached cone 6; it points to glaze application and suspension, not temperature. The next step is to measure specific gravity and adjust to the studio's target (commonly 1.40-1.45 for dipping), sieve the batch through an 80-mesh sieve to break up agglomerates, and let it slake before re-testing. Do not simply add more dry mix; correct one variable at a time and re-dip a tile.
2. The other student asks whether to dip the platter longer or add water. What do you advise, and how do you verify?
Read the worked answer
Neither, until the batch is corrected. Dipping longer on a thickened, poorly dispersed slurry builds a lumpy, uneven layer that pinholes and can run; adding water lowers specific gravity and thins the layer further, undoing the gain. Advise the student to wait for the sieved, gravity-adjusted batch, then dip the platter once at the corrected gravity and measure the dry layer with a needle gauge at several points. Target 0.4 mm; if a spot reads low, a second quick dip after the first dries is safer than a long soak. Fire a small test tile alongside the platter so any surprise is caught on scrap, not on the large piece.