Specific Gravity: Getting the Right Amount of Water in Your Glazes

When we mix up a batch of glaze, we start with dry powders and add water to create a slurry that we can apply to our pots. Getting the water content just right is crucial - too much water and the glaze will be runny, drip, and apply too thinly; too little water and it will be thick, difficult to apply evenly, and possibly lead to application issues. Even when following a recipe, variables like humidity, bisque temperature, and application method mean we need a reliable way to check and adjust the water content in our glazes.

What is Specific Gravity?

Specific gravity (SG) is simply the weight of your glaze compared to water. Since water has a specific gravity of 1.0, a glaze with a specific gravity of 1.5 is 1.5 times heavier than the same volume of water. The higher the specific gravity, the less water and more solids in your glaze.

Getting the specific gravity right helps you:

• Apply glaze evenly
• Prevent dripping and running
• Achieve consistent results from batch to batch
• Control glaze thickness

What is the “right” Specific Gravity?

The “right” specific gravity is highly dependent upon the glaze, application method, bisque porosity and other factors.

In general, a specific gravity of 1.4-1.5 is a good starting point for most glazes, but you may need to adjust to anywhere from 1.25-1.6 for your needs. Casting slips will have much higher specific gravities, often 1.75-1.8.

For clear glazes where you want a thin coat, you may find that a lower specific gravity is better, while some glazes may need a thicker coat to achieve the desired effect. As you adjust the specific gravity of a glaze batch, you can test application on a small piece of bisque or test tile. Once you’ve “dialed in” the specific gravity for that glaze, make a note of it for future reference.

How to Measure Specific Gravity

There are two common ways to measure specific gravity, using weight and volume and using a hydrometer.

Weight and Volume Method

Measuring with a Syringe (Recommended)

1. Zero-out the empty syringe on the scale. It’s easiest to use a 100ml syringe, but any size will work.

2. Draw up exactly 100ml of glaze into the syringe.

3. Weigh the syringe and divide by the volume. In this example, the weight of the glaze is 148.1 grams, so the Specific Gravity is 148.1 / 100 = 1.48.

Measuring with a Beaker

1. Adjust the specific gravity of the glaze. Here, a 250ml beaker is zeroed-out.

2. Fill the beaker with glaze up to a specific mark. In this example, we fill to 250ml. The weight of 250ml glaze is 386.7. Dividing by the weight of water, 386.7/250 = 1.55. For this glaze, 1.5-1.6 is a good pouring thickness. (Using a 100ml sample it’s even easier to calculate the specific gravity, in this example the weight would be 155g with specific gravity of 1.55.)

Hydrometers

A hydrometer  is a floating tool with a scale that measures specific gravity directly. While convenient, they don’t work well in thick glazes and can be less accurate than the weight method.

Why Hydrometers can be problematic:

• You need a tall container and a good amount of glaze, the hydrometer needs to float and bob freely
• Most ceramic glazes are too viscous for accurate readings, and thixotropic (gelled) glazes prevent free movement
• Hydrometers designed for liquids lighter than water like beer & wine won’t work
• Different scale types (Baumé vs. specific gravity) cause confusion

Even with these caveats, some potters still find that hydrometers work for them. If you want to try one, look for a hydrometer from a ceramics supply store that is designed to work for glazes.

Adjusting Your Glaze

If specific gravity is too high:

• Add small amounts of water and mix thoroughly
• Re-test after each addition

If specific gravity is too low:

• Let some water evaporate
• Add more dry glaze materials

References

• Digitalfire: Specific gravity 
• Video: Glaze Thickness or Specific Gravity