Almost every glaze fault blamed on the recipe is actually a thickness fault. A glaze that pinholed at 1.4 specific gravity is fine at 1.45; a glaze that crawled at 0.8 mm is fine at 0.45 mm. This is how to measure glaze thickness properly: a hydrometer for the bucket, a scale for the test tile, and the numbers that make a glaze repeatable from one session to the next.
H 9 · Ø9 / Ø9.5 / Ø5 cm
The bucket: specific gravity and flocculation
A dipping glaze should sit at 1.42 to 1.48 specific gravity for most cone 6 recipes. Measure with a glaze hydrometer, not with a guess based on how the glaze drips off a stick. Below 1.40 the glaze is too thin and you will need three dips for the thickness of one; above 1.52 it is too thick and will crawl.
If the glaze settles hard within an hour of stirring it needs flocculating: add 0.2 percent Epsom salts by dry weight, dissolved in warm water first. Flocculated glaze stays in suspension for days and — more importantly — holds a thicker layer on the pot per dip, which is what makes the difference between 0.3 mm and 0.5 mm from a one-second dip.
The dip: one second, and how much it deposits
A one-second dip in a well-flocculated cone 6 glaze at 1.45 specific gravity deposits roughly 0.35 to 0.45 mm of dry glaze, which corresponds to about 45 to 60 grams per 100 square centimetres of surface. That is the target range for most transparent and satin glazes.
Time the dip with a count rather than a feel. One-and-a-half seconds is measurably thicker than one second, and the difference shows up as a run at the base of a pot. Keep the pot moving slowly as it goes in and comes out so the glaze does not build up a line where the surface breaks.
The scale test
The only way to know your thickness is to weigh a test tile. Take a bisqued tile, weigh it dry, dip it for one second, dry it for an hour and weigh it again. The difference divided by the tile area gives grams per 100 square centimetres. For a 5 by 5 cm tile, 0.25 g of glaze means 100 g per 100 square centimetres — far too thick; 0.12 g means 48 g per 100 square centimetres, which is right.
Do this once per bucket and write the number on the bucket lid. When a glaze starts behaving differently three weeks later, the first thing to check is whether the deposit weight has changed, and the second is the specific gravity. Those two numbers explain most glaze variation.
Thickness by glaze type
Clear and transparent glazes want 0.35 to 0.45 mm. Satin mattes want slightly less, 0.3 to 0.4 mm, because they break over edges when applied thickly. Iron saturates and tenmoku want more, 0.6 to 0.9 mm, because the crystal development depends on the glaze being able to move. Copper reds are the most sensitive: 0.4 to 0.5 mm gives red, 0.7 mm gives a muddy grey-green, and the difference is invisible before firing.
The inside of a drinking vessel is the exception to all of this. Interior glaze should be 0.3 to 0.4 mm and even, because a thick interior glaze on a vessel that holds hot liquid will craze from thermal cycling even if it is a perfect fit at room temperature.
Steps
- Stir the bucket thoroughly and measure the specific gravity with a glaze hydrometer; adjust with water to 1.42 to 1.48.
- Add 0.2 percent Epsom salts dissolved in warm water if the glaze settles hard within an hour.
- Weigh a bisqued test tile dry and record the weight.
- Dip the tile for exactly one second, moving it slowly in and out, then dry it for an hour.
- Weigh the tile again and compute grams per 100 square centimetres from the area of the tile.
- Adjust the dip time to land in the 45 to 60 g per 100 square centimetres range.
- Repeat the tile test for each glaze type, targeting 0.3 to 0.4 mm for mattes and 0.6 to 0.9 mm for iron saturates.
- Write the deposit weight and the specific gravity on the bucket lid, and re-check both every few weeks.
Where it goes wrong
- Judging thickness by the colour of the raw glaze on the pot. Some glazes look opaque at 0.3 mm and others look thin at 0.6 mm.
- Adding water to a settled glaze without stirring it back into suspension. You get a thin top layer and a thick bottom layer in the same bucket.
- Using the same dip time for every glaze. Copper reds and iron saturates need opposite extremes.
- Weighing the tile before it is fully dry. An hour of drying at room temperature is the minimum; a damp tile reads heavy.
| Symptom | Cause |
|---|---|
| Glaze crawled in patches | applied over 0.6 mm on a matte recipe that shrinks differently from the body. |
| Pinholing across the whole surface | glaze too thick, so the surface sealed before the body stopped outgassing. |
| Copper red came out muddy grey-green | applied too thick; the reduction at the surface was not strong enough to keep the copper in the red state. |
| Glaze ran off the foot and stuck to the shelf | dip time increased without re-checking the deposit weight. |
Recipe and schedule numbers here were recorded on the workshop’s own kiln. A different kiln, a different thermocouple position or a different clay batch will move the result, so treat every firing schedule as a starting point and keep your own log beside it.
Same cone — Cone 6
- Centering a 1 kg ball: wheel speeds, hand positions and the wobble test Stoneware
- Reading a glaze fault: six defects and what actually causes them Stoneware
- Opening and flooring: how thick the base actually needs to be Stoneware
- Rolling slabs to an even 8 mm and joining them so the seam outlives the pot Stoneware
- Spiral wedging a 5 kg batch without trapping air Stoneware
- Throwing a 24 cm cylinder from 1.5 kg: the proportions that keep it standing Stoneware