Ceramic Glaze Specific Gravity: What It Is & How to Measure It | B2W Ceramics
Ceramic Glaze Specific Gravity: What It Is and How to Measure It
Specific gravity is one of the simplest numbers you can track in your studio — and one of the most useful. If you mix your own glazes, it gives you a repeatable baseline so your results stay consistent from one firing to the next. This post covers what specific gravity actually measures, four ways to check it, and — importantly — what it can’t tell you.
What Specific Gravity Actually Measures
Specific gravity (SG) is the density of your glaze relative to pure water. Water sits at 1.0. A glaze with an SG of 1.45 is 1.45 times denser than water — that extra density comes from the dry materials suspended in solution.
What this means practically: SG is a proxy for how much dry material is in your glaze relative to water. Let water evaporate from your bucket and the SG climbs — your glaze goes on thicker. Add too much water and it drops — coverage thins, color shifts, and your results become unpredictable.
If a glaze fired beautifully at 1.40, you can return to that number every time and expect consistent behavior. That’s the value.
A Starting Point for New Glazes
100g dry materials : 80g (or ml) water. This gives you a workable dipping consistency and a concrete reference point to adjust from. Measure the SG immediately and write it down — that number becomes your anchor as you refine the recipe.
Four Ways to Measure Specific Gravity
All four methods are valid. Which one you use comes down to what equipment you have and how you prefer to work.
01 — Hydrometer
You’ll need:Hydrometer · Graduated cylinder (if your bucket is too shallow)
Lower the hydrometer into your glaze and let it settle. Tap it gently to break any surface tension, then read the number at the waterline. Fast, no math required — but hydrometers are fragile and the scale can be difficult to read accurately in thick slips.
Tare your empty cylinder, then fill it to exactly 100ml with your glaze. The reading in grams divided by 100 is your SG. This works because 100ml of water weighs exactly 100g — any additional mass reflects the density of your suspended materials. Reliable and precise, though it uses more glaze than the syringe method.
Tare the empty syringe, draw your glaze to the 100ml mark, and weigh it. Divide by 100. Same math as the cylinder method, but the syringe is easier to handle with thick glazes and wastes less material. A good option if you’re testing small batches.
Fill the cup completely, place the lid so excess glaze is displaced through the hole, and weigh the result. Divide by the cup’s known volume to get your SG. These cups are purpose-built for the task and handle well in production contexts — worth having if you’re running a lot of glazes regularly.
What Specific Gravity Can’t Tell You
SG tells you how much material is in your glaze. It says nothing about how your glaze flows.
That’s viscosity — and the two are not the same thing. Two glazes at identical specific gravities can behave completely differently when you dip a piece. One flows off cleanly; the other drags and grabs. The difference comes down to flocculation, particle size, the specific materials in your recipe, and how long the glaze has been sitting.
SG tells you what’s in the bucket. Viscosity tells you how it behaves. For truly consistent results across firings, you need both numbers.
Viscosity — how to measure it and how to adjust it — is covered in the next post.
