Uploaded April 2026 | Updated September 2026, 2 weeks ago
It's probably the most popular glaze attribute to measure, but I'm not convinced that it's particularly useful (at least, not on its own).
It is literally just a measure of density. I don't think there are too many people who care about glaze density itself. Where it is useful is that it can be a shorthand for added water and for viscosity.
A low specific gravity means there's a lot of added water and a low viscosity. This means dipped glaze applications will be thinner, and building up any thickness will be difficult and more likely to cause crawling.
A high specific gravity means there's less water and a higher viscosity. This means dipped applications will be thicker. Crawling is less likely for a given thickness of glaze (less drying shrinkage), but more likely overall because the application will be thicker for a given dip time.
The correlation between added water, specific gravity, and viscosity is unique to each glaze recipe and source of ingredients. This means you can use the number to track your glaze behaviour over time, but you can't meaningfully apply someone else's specific gravity number.
In my opinion, it's the glaze viscosity that's the key to glaze applications. Some glazes gel over time, so they get more viscous for the same water content. A viscous glaze will dip differently to a fluid glaze regardless of what the specific gravity does. If your glaze is getting too viscous to dip well, you can dilute it with water (lowering specific gravity) or deflocculate it (makes it more fluid for the same specific gravity). Unless you have a specific reason to do the second option, it's far simpler to do the first. Regardless of what you do, if you can end up with the same dry glaze equivalent on the piece then you'll get the same fired result.
That's specific gravity in a nutshell. I believe it's overrated and misapplied most of the time, but it's absolutely crucial in some cases. If you're using a deflocculant, specific gravity is really important.
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Support the creation of more videos on Patreon: patreon.com/oldforgecreations
For examples of my work that's been glazed, check out my Instagram:
instagram.com/oldforgecreations
Buy my work, or read more on my blog:
oldforgecreations.co.uk/shop
Link to tools: oldforgecreations.co.uk/recommended-tools
It's probably the most popular glaze attribute to measure, but I'm not convinced that it's particularly useful (at least, not on its own).
It is literally just a measure of density. I don't think there are too many people who care about glaze density itself. Where it is useful is that it can be a shorthand for added water and for viscosity.
A low specific gravity means there's a lot of added water and a low viscosity. This means dipped glaze applications will be thinner, and building up any thickness will be difficult and more likely to cause crawling.
A high specific gravity means there's less water and a higher viscosity. This means dipped applications will be thicker. Crawling is less likely for a given thickness of glaze (less drying shrinkage), but more likely overall because the application will be thicker for a given dip time.
The correlation between added water, specific gravity, and viscosity is unique to each glaze recipe and source of ingredients. This means you can use the number to track your glaze behaviour over time, but you can't meaningfully apply someone else's specific gravity number.
In my opinion, it's the glaze viscosity that's the key to glaze applications. Some glazes gel over time, so they get more viscous for the same water content. A viscous glaze will dip differently to a fluid glaze regardless of what the specific gravity does. If your glaze is getting too viscous to dip well, you can dilute it with water (lowering specific gravity) or deflocculate it (makes it more fluid for the same specific gravity). Unless you have a specific reason to do the second option, it's far simpler to do the first. Regardless of what you do, if you can end up with the same dry glaze equivalent on the piece then you'll get the same fired result.
That's specific gravity in a nutshell. I believe it's overrated and misapplied most of the time, but it's absolutely crucial in some cases. If you're using a deflocculant, specific gravity is really important.
----
Support the creation of more videos on Patreon: patreon.com/oldforgecreations
For examples of my work that's been glazed, check out my Instagram:
instagram.com/oldforgecreations
Buy my work, or read more on my blog:
oldforgecreations.co.uk/shop
Link to tools: oldforgecreations.co.uk/recommended-tools
