Percentages to a batch
Glaze recipes are written as percentages because that makes them portable: the same recipe scales to a test tile or a five gallon bucket without rewriting. To use one, pick a dry batch weight and multiply each percentage by it. A recipe with 25 percent feldspar scaled to a 2,000 gram batch takes 500 grams of feldspar. That is the whole arithmetic for the dry side.
The convention that trips people up is additions. Colorants, opacifiers and suspending agents are normally written above the base 100 rather than inside it: a recipe listing five materials summing to 100 followed by "plus 3 percent red iron oxide" means 3 percent of the base weight added on top, so the batch actually totals 103 percent. This page treats the last two lines as additions for that reason and applies every percentage to the base weight as written, without normalising. If your base lines do not sum to 100, that is flagged rather than silently corrected, because the correct response depends on whether it was intentional.
Batch size and what it costs you to be wrong
Test batches exist because a recipe from a book, a forum or another studio is not the same glaze in your kiln on your clay. The materials differ between suppliers, your firing schedule differs, and the body underneath changes the result. A 100 to 500 gram test batch on tiles of your own clay, fired in your own kiln, is the only way to find out. Scaling straight to a bucket because a photograph looked good is how studios end up with twenty litres of a glaze nobody wants to use.
| Batch | What it is for | Scale accuracy needed |
|---|---|---|
| 100 g | First look, a few test tiles | 0.01 g, since a 2% colorant is 2 g |
| 500 g to 1 kg | Enough to dip a few pots and judge it properly | 0.1 g |
| 5 kg | A working bucket for a small studio | 1 g |
| 10 kg and up | Production quantity | 1 g, and weigh into the bucket cumulatively at your peril |
The accuracy column matters more than it looks. A 2 percent colorant addition in a 100 gram test batch is 2 grams, and a scale reading to whole grams turns that into anywhere from 1.5 to 2.5 — a 25 percent error in the thing that determines the colour. Small batches need a better scale, not a rougher one.
Water, specific gravity and what actually controls thickness
The water calculation here is real arithmetic and it is still only a starting point. Given a dry weight, an average particle density for the mix, and a target specific gravity, the water needed follows directly, because specific gravity is total mass divided by total volume and both are known once the water is chosen. What makes it an estimate rather than an answer is the particle density: 2.5 is reasonable for a typical silicate recipe, but a glaze loaded with zirconium or tin opacifier is denser and one heavy in light clays is not.
So mix with about four fifths of the calculated water, sieve, and then measure. Measuring is easy: weigh exactly 100 millilitres of the mixed glaze in grams and divide by 100. A reading of 145 grams is a specific gravity of 1.45. Add water in small increments and re-measure, because it is trivial to add water and tedious to remove it — thinning a bucket that went too far means settling it overnight and pouring off the clear water on top.
What specific gravity controls is roughly how much solid is deposited per second of dip, and it is the main lever you have. It is not the only one. A deflocculated bucket at 1.45 behaves nothing like a flocculated one at the same reading, dry bisque pulls a heavier coat than bisque that has been handled or fired higher, and a three second dip is not the same as a one second dip. Record the specific gravity together with the dip time and the bisque cone, because those three together are the recipe for the application, and the number in the notebook is worth more than any calculation.
Sieving, settling and hard panning
Every glaze wants sieving, usually through 80 mesh at minimum and finer for smooth surfaces, and twice through is standard practice. It breaks up agglomerated material that no amount of stirring will disperse, and it catches contamination. A glaze that has not been sieved will show it as specks and pinholes that look like a firing fault.
Settling is the other everyday problem. Glazes high in feldspar and low in clay settle into a hard layer at the bottom of the bucket that a stir stick cannot recover, and the usual response is a small addition of bentonite in the dry batch or an epsom salt solution added to the wet bucket to flocculate it. Both change how the glaze applies as well as how it sits, which is another reason the specific gravity you settle on is one you arrive at with the bucket in front of you rather than one you calculate in advance.
Questions people ask
How do I scale a glaze recipe to a specific batch size?
Multiply each percentage by the batch dry weight and divide by 100. For a 5,000 gram batch, a material listed at 18 percent is 900 grams. If the recipe includes additions written above the base 100, such as a colorant at 4 percent, apply that to the same base weight rather than to a normalised total, so the 4 percent addition on a 5,000 gram base is 200 grams and the batch totals 5,200 grams of dry material. Do not normalise the additions into the base unless the recipe was clearly written that way, since a recipe totalling 104 percent is normal and a recipe totalling 97 percent is usually a typo.
What specific gravity should a dipping glaze be?
There is no universal number, which is why this page takes it as an input. Dipping glazes commonly land somewhere in the region of 1.4 to 1.5, but the right figure is whatever gives the coat thickness that recipe needs on your bisque, and glazes vary widely in what that is. A glaze intended to go on thin will want a lower reading than one that develops its character in a thick application. The productive approach is to start near the middle of that range, dip test tiles, fire them, and adjust — then write the number that worked in a notebook next to the recipe. That recorded figure is the useful one; the calculated estimate is only how you get to the first bucket.
How do I measure specific gravity without a hydrometer?
With a scale and a measuring cylinder or any container whose volume you know exactly. Stir and sieve the glaze thoroughly, measure out exactly 100 millilitres, and weigh it in grams. Divide by 100 and that is the specific gravity: 142 grams means 1.42. Tare the container first, or weigh it empty and subtract. This is more reliable than a hydrometer in a thick glaze, where a hydrometer can be dragged by the suspension and read high. Whatever method you use, use the same one every time, since consistency between readings matters more than absolute accuracy.
Is a glaze food safe if the recipe says so?
A recipe cannot tell you that, and this page will not assert it about anything. Whether a fired glaze releases metals into food or drink depends on the fired result rather than the ingredient list: the actual chemistry after firing, whether the glaze matured properly in your kiln, and the state of the surface. The same recipe underfired, crazed or applied over a different body can behave completely differently. Certain colorants and fluxes attract particular scrutiny on functional ware. Answering the question properly means a leach test at a laboratory on the actual fired ware, which is the only route that produces an answer you can rely on.
Why does my glaze settle into a hard layer at the bottom?
Because there is not enough clay in the recipe to keep the heavier particles in suspension, and the mix has flocculated into a dense pan rather than staying loose. Recipes high in feldspar, silica and frit with little kaolin are the usual candidates. The common corrections are a small bentonite addition mixed into the dry materials before water goes in — it must be dispersed dry or it forms lumps — or an epsom salt solution added to the wet bucket a few drops at a time to flocculate the suspension. Both change the application characteristics as well as the settling, so re-check the specific gravity and dip a test tile afterwards rather than assuming the bucket behaves the same.