If you mix your own glazes, you’ve probably measured specific gravity before. But did you know you can use that number to calculate exactly how much dry material is suspended in your glaze? This is useful for scaling recipes, checking consistency, and understanding how your glaze will behave in the kiln.
What Is Specific Gravity?
Specific gravity (SG) is the ratio of your glaze’s density compared to pure water. Water has an SG of 1.0. A glaze with an SG of 1.45 is 1.45 times denser than water — that extra density is your dry material.
The Key Insight
When you mix a glaze, you’re suspending dry material in water. The water component always weighs the same as its volume — 1ml of water weighs 1g. So if you know the total weight of your glaze and subtract the weight of the water, what’s left is your dry material.
The Formula
Total glaze weight = SG × Volume (ml)
Water weight = Volume (ml)
Dry material weight = Total glaze weight − Water weight
A Simple Example
Let’s say you have 1000ml of glaze with a specific gravity of 1.45.
- Calculate total glaze weight:
1.45 × 1000ml = 1450g - Identify the water weight:
1000ml of water = 1000g - Subtract to find dry material:
1450g − 1000g = 450g of dry material
That means in every 1000ml of that glaze, 450g is dry material and 1000g is water.
Dry Material as a Percentage
You can also express this as a percentage of the total glaze weight:
450 ÷ 1450 × 100 = 31% dry material by weight
This is useful when comparing glazes or diagnosing application problems.
Quick Reference Table
Here’s how much dry material is in 1000ml of glaze at common specific gravity readings:
| Specific Gravity | Total Weight (1000ml) | Dry Material | % Dry Material |
|---|---|---|---|
| 1.30 | 1300g | 300g | 23% |
| 1.35 | 1350g | 350g | 26% |
| 1.40 | 1400g | 400g | 29% |
| 1.45 | 1450g | 450g | 31% |
| 1.50 | 1500g | 500g | 33% |
Why This Is Useful
Knowing your dry material content helps you in several practical ways:
- Scaling up a batch — Calculate exactly how much dry material you need to add to reach a target SG
- Diagnosing application problems — Too thin or too thick? Now you have numbers to work with
- Comparing glazes — Understand why one glaze applies differently than another
- Reclaiming glaze — Know how much material you’re working with before adding water back in
The Takeaway
Specific gravity isn’t just a consistency check — it’s a window into the chemistry of your glaze. With a volume measurement and your SG reading, you can calculate exactly what’s in your bucket. Once you start thinking about glazes this way, troubleshooting becomes a lot less guesswork.
