How and Why We Measure Specific Gravity in Our Glaze Studio
One of the most important goals in any pottery studio is consistency. When you dip a pot into a glaze, you want to know that it will apply the same thickness today as it did last week and that it will fire the same way next month as it does today.
Unfortunately, glazes are constantly changing. Water evaporates, materials settle, and well-meaning potters occasionally add water when a glaze feels too thick. Over time, those small changes can dramatically affect how a glaze applies and fires.
That's why we routinely measure specific gravity at Blue Dog Ceramics. It's a quick process that allows us to monitor every glaze bucket and maintain consistent results throughout the studio.
What Is Specific Gravity?
Specific gravity is a measure of the density of a glaze compared to water.
Water has a specific gravity of 1.00. Since glazes contain suspended materials such as clay, silica, feldspar, frits, and colorants, they are heavier than water and therefore have a specific gravity greater than 1.00.
For example, if 100 mL of glaze weighs 147 grams, the glaze has a specific gravity of 1.47.
The exact number isn't as important as maintaining the same number over time. Every glaze has its own ideal specific gravity based on how it applies and fires.
Why We Measure Specific Gravity
As a glaze bucket is used, water slowly evaporates. As the water content decreases, the concentration of solids increases, making the glaze thicker.
A thicker glaze often results in:
Heavier glaze application
Longer drying times
Increased running during firing
Changes in color and surface quality
If too much water is added, the opposite happens. The glaze becomes too thin and may produce weak color, poor coverage, or uneven surfaces.
Rather than relying on how a glaze looks or feels, we use specific gravity to make objective, repeatable decisions. Measuring allows us to maintain the same glaze consistency month after month, regardless of who is using the bucket.
How We Measure Specific Gravity
Our method is simple, repeatable, and requires only a few inexpensive tools.
Equipment
Digital scale capable of reading grams
100 mL plastic syringe
Spreadsheet or notebook for recording measurements
Step 1: Tare the Scale
Place the empty 100 mL syringe on the digital scale and press the tare button so the display reads 0 grams.
This removes the weight of the syringe from the measurement, allowing the scale to display only the weight of the glaze.
Step 2: Thoroughly Mix the Glaze
Before taking a sample, thoroughly mix the glaze.
Many glaze ingredients settle to the bottom of the bucket over time. Scrape the bottom and sides while mixing to ensure the glaze is completely homogeneous before collecting a sample.
Step 3: Draw a 100 mL Sample
Using the syringe, slowly pull the plunger until you have exactly 100 mL of glaze.
Because every sample is the same volume, each measurement can be directly compared with previous measurements.
Step 4: Weigh the Sample
Place the loaded syringe on the tared scale.
The number displayed is the weight, in grams, of 100 mL of glaze.
For example, if the scale reads 147 grams, the glaze has a specific gravity of 1.47.
Record this value.
At Blue Dog Ceramics, we keep these measurements in a spreadsheet so we can monitor each glaze over time, but a written glaze log works just as well.
Step 5: Return the Sample
After recording the measurement, simply empty the syringe back into the glaze bucket.
Nothing is wasted, and the glaze bucket is ready to use.
Using the Measurement
Once you've established the ideal specific gravity for a glaze, every future measurement becomes a comparison against that standard.
If the measured value is higher than your target, water has evaporated and the glaze has become more concentrated. Add a small amount of water, mix thoroughly, and measure again until you return to the target specific gravity.
If the measured value is lower than your target, too much water has been added. Depending on the situation, you may allow some water to evaporate naturally or remove excess water from the top of the settled glaze before mixing again.
The key is to make small adjustments and remeasure after each one.
Consistency Creates Better Pottery
Pottery is full of variables that we can't completely control, but glaze consistency doesn't have to be one of them.
Measuring specific gravity takes only a few minutes, yet it removes much of the guesswork from glazing. By maintaining a consistent specific gravity, we know our glazes will apply more predictably, fire more consistently, and produce results our students can trust.
Like many good studio habits, it's a simple process—but one that pays dividends every time the kiln is opened.
