Why Specific Gravity Isn’t Listed on Glaze Recipes
If specific gravity is so important, why isn’t it listed on glaze recipes?
If you’ve been glazing for any length of time and especially if you’ve been following my teachings, you may have heard about the importance of measuring the specific gravity of your glazes.
And if you’re new to measuring specific gravity (and you haven’t yet taken my Mastering Glaze Consistency course) you may have found yourself wondering:
“If it’s so important to measure specific gravity, why aren’t specific gravity values published on glaze recipes?”
This is a very good question and there are a few reasons for this that I’ll explain in this article.
Let’s start with a little bit of science and math
In case you’re not familiar with what specific gravity is, it’s a measurement you can take that tells you the proportion of solid particles to water in your glaze bucket. This ratio is important because it affects the thickness of your glaze application, which has a major impact on your fired results.
In every glaze bucket, you have solid particles suspended in water. The solid particles start as dry powders and we mix them with water to create a suspension.
A suspension is a term that describes a fluid that contains solid particles that do not dissolve, but remain solid and free to float around the liquid (are “suspended” in the liquid) and will eventually settle out on the bottom of the container if undisturbed.
A suspension is not to be confused with a solution, which describes a fluid in which solid particles dissolve and changed into a liquid state (like dissolving Epsom salts in water to create a saturated Epsom salt solution for flocculating your glazes).
So, in every glaze bucket, we have solid glaze particles suspended in water. The glaze particles have weight (mass) and take up space (volume), and the water has weight and takes up space.
The relationship between mass and volume is called density.
Mass (g) ÷ volume (mL) = density (g/mL).
The density of pure water is constant at 1 g/mL.
The density of a glaze (solids plus water) will fall somewhere between 1 and 2 g/mL. It’s higher than 1 because the solid particles in the glaze are heavier (denser) than water.
Since we know that the density of water is always 1, we can use this value to understand how much water is in our glaze bucket. The more water we have in the glaze bucket, the closer the density will be to 1.
The specific gravity of a fluid refers to the density of that fluid, relative to the density of water. To calculate specific gravity, you divide the density of the fluid by the density of water. This is a very simple calculation since the density of water is 1. So as far as we’re concerned, the density and specific gravity of a glaze are equivalent and have the same value.
For the rest of this article, I’ll use the term specific gravity but I could also say density and be talking about the same thing. The terms are interchangeable for our purposes.
If we want to understand the proportion of solid particles to water in any sized glaze bucket at any given time, we can measure specific gravity by weighing a volume of glaze and dividing the mass in grams by the volume in millilitres.
If you want to see the steps for how to measure specific gravity of your glazes, you can request my free guide that will show you what you need and how to do it.
Why bother measuring specific gravity?
So now that we know what specific gravity is, let’s talk about why we should measure it.
The amount of water in your glaze bucket should be thought of as a proportion of the glaze in the bucket. Your entire glaze bucket is part solids and part water.
When the ratio of solids to water is high (more solids, less water) your specific gravity will be high and your glaze will apply more thickly than if the ratio of solids to water is low (less solids, more water) and your specific gravity is low.
Have you ever glazed a piece where you’ve followed the instructions and held the piece in the glaze for the exact recommended # of seconds and your glaze either ran or crawled because it was too thick, or it was too thin and the surface felt rough or the glaze just looked like crap?
You have? Me too!
That’s because controlling your dip length means nothing if your water content is variable. A 3 second dip is only equivalent to another 3 second dip if the water content (specific gravity) is the same.
Both test tiles held in the same glaze for the same number of seconds. Different water content = different results
So giving instructions for how long to hold a piece in a glaze is basically irrelevant without a specific gravity value.
If you want to test this, grab a piece of bisque and dip it in your glaze bucket for exactly 3 seconds. Then dump a whole bunch of water into your glaze, mix well, and dip another equally sized/shaped piece in the glaze for exactly 3 seconds.
You probably don’t need to actually do this test to understand how the results will be different. The piece dipped in the glaze with the extra water added will have a thinner glaze coat than the first piece.
When you can attach a numerical value to the amount of water in your glaze bucket by measuring specific gravity and making sure it’s always the same, then your efforts to keep your application methods consistent will give you consistent application thickness and consistent fired results.
What should my glaze’s specific gravity be?
Hopefully by now you’re convinced that controlling your glaze’s water content will lead to more consistent glaze results. But what should the specific gravity be??? What’s the right number?
The answer is, it depends!
There are so many factors that affect how your glazes will turn out that there couldn’t possibly be one all encompassing answer for specific gravity.
It depends on the size and shape of your pieces. If you only make large pieces that take longer to dip, you may want to keep your SG lower.
It depends on your application methods. A different SG will be ideal depending on whether you’re dipping, brushing or spraying your glazes.
It depends on your bisque temperature. Clay that’s bisque-fired to a lower temp can absorb more glaze water than a higher bisque temp.
It depends on whether you’re using the glaze by itself or layered with other glazes. If you’re layering glazes, you may want a lower SG than someone who uses the same glaze by itself.
It depends on how you want the glaze to look. This is the all encompassing answer. The specific gravity you keep your glaze at is totally up to you and how you like your glaze best.
The right specific gravity for you might not work for someone else because we all make different pots and have different desired results.
“Please just tell me the answer!”
Ok, for those of you who are dying to know, most dipping glazes that I’ve used work best somewhere between 1.4 – 1.6. Not every glaze, but most of them.
So many variables
Since the ideal specific gravity for a glaze depends on so many variables, this is one reason why it’s not included on glaze recipes.
Specific gravity isn’t a “set it and forget it” type of process. If that was the case, we could just include water as a percentage on the recipe and we’d be good to go. But water is a variable. It evaporates and needs to be replaced.
The glazing process, bisque absorption and how well you mix your glazes can cause the water content to fluctuate. So even if you start with a set amount of water, weeks/months down the road it’s likely to have changed.
Specific gravity always needs to be checked and adjusted. Measuring it will tell you what’s in the bucket right now.
I would much rather you figure out through testing, the best specific gravity for your glazes on your clay bodies at your temperature with your application methods.
Even if you’re using my glaze recipes, the specific gravity I use might not be the best for you.
I used my testing process to determine the best specific gravity for a collection of glazes that were being used in a high volume studio with a lot of beginners who were mostly dipping. If I were using these same glazes on my own pots at home, I may choose different specific gravity values for them.
The key here is that I would test them and then decide. And that’s what I want to encourage you to do as well. When you understand the nuances of the water levels in your glazes, you become a better glazer and problem solver. Your intuition about glaze gets stronger and you can adapt to variables that you encounter.
Not everyone measures specific gravity
Another reason for the omission of SG on recipes, and possibly the main reason, is that not everyone measures specific gravity. It isn’t really a widespread common practice. Nobody taught it to me. I sought out the info in order to solve some glaze consistency challenges I was having while running a busy studio.
I had heard of it, I Googled it and read as much info as I could find on it (which wasn’t much). I started implementing what I learned, I made a lot of mistakes, and over time I developed a system that really works. So now, that’s what I teach.
But if you go talk to potters who have been glazing for 30+ years, a lot of them probably don’t measure specific gravity or think it’s worth their time. Many believe that glazing is an art form and the right amount of water can be determined intuitively. And I completely respect that. I’m not out to change what’s working for people. I’m just out to share what works for me.
I have a very analytical brain that loves science, numbers and problem solving, which is why I’ve found the specific gravity calculation to be extremely useful. Having all the data from testing over and over again has given me the intuition that wasn’t natural for me to begin with.
Since many published glaze recipes were developed by people who weren’t in the practice of measuring specific gravity, this is a simple explanation for why it’s not included on published recipes.
Find the best specific gravity for each of your glazes
You can figure out the best specific gravity for your glazes and include it in your own notes and on the recipes you’re using. The best way to get started is to use your intuition to adjust the water content as you’ve likely always been doing.
Once you get your glaze to how you would normally use it, measure specific gravity. Write that number in your notes with the date. Proceed with glazing and firing. When you unload the kiln, go through all of your fired glaze results and make notes about which ones seem great, which ones seem too thin and which ones seem too thick.
Attach your “thin/thick/just right” assessment to the specific gravity value you had calculated before glazing and then make adjustments based on your desired results. When you get a glaze that’s just right, make sure the SG is always the same every time you glaze.
Mastering Glaze Consistency
If you want to learn everything I have to teach about specific gravity including my testing method to determine the best conditions (water content, viscosity, flocculation, bisque temp, etc) for your glazes so you can have consistent results firing after firing, this is what I teach in my online course Mastering Glazy Consistency. Join the waitlist for the next enrollment!
Learn to Mix Glazes from Scratch
If you’ve never mixed a glaze from scratch before and want to learn, I teach an online workshop called Glaze Mixing Essentials where I show you all the steps to mix a glaze and then test a base glaze with multiple colourants. Click the link for all the details.
Join my free community
If you love learning about and discussing glazes, I'd like to invite you to my free social learning Facebook group called Understanding Glazes with Sue. The group is full of videos and discussions about firing, mixing glazes and fixing various glaze issues. Please join!