How to Measure the Specific Gravity of Your Glazes
Video Tutorial
In this video I demonstrate how to measure the specific gravity of a glaze I just mixed using a graduated cylinder and a scale.
Measuring specific gravity is a way to calculate the water content of a glaze to ensure that each time you glaze, you have the same amount of water.
CONSISTENT WATER CONTENT = CONSISTENT APPLICATION THICKNESS = CONSISTENT RESULTS
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Thank you so much, Sue, for your thorough demonstration of establishing SG. I use a tall skinny plastic bottle, the bottom filled with heavy large construction nails. I cap the bottle and store it so the weight inside does not shift. I have marked my preferred viscosity on the bottle (after submerging it in my glaze and seeing the outcome on the fired pots) I try to create this same level of float at my mark each time I mix more glaze into this same existing glaze bucket…yes, a very primitive way of checking SG.
You have encouraged me to use a scale and graduated cylinder… !!!!
Hi Marsha,
Sounds like you’ve made yourself a homemade hydrometer! Very innovative. I hope you find using a scale and graduated cylinder a little more precise.
Sue
Hi Sue! Great stuff here! I have a suggestion for you that you may already know by now. I use a plastic 60 mL veterinary syringe when testing SG for smaller amounts of glaze. Easy to find at farm stores or Amazon. Also for really small batches I use a 10 mL syringe. Easy to squirt the contents back into the batch. I also find the graduated cylinder difficult to clean out, and the plastic syringes are much easier to work with. Also my little electronic balance turns itself off rather quickly, so I no longer tare the syringe, but write the weight on it with a sharpie. Then I can simply subtract the weight of the syringe.
I hesitate to remove water as you do, because there are likely some dissolved components that you are losing when you remove the water. I’m thinking that if you were to boil away the water you’d be able to add that dry residue back to the glaze.
I’ve taught chemistry at community colleges for over 20 years and am having great fun playing with glaze chemistry!
Hi Debbie, these are great tips, thank you! I hear a lot of people use a syringe. I don’t have a decent one so haven’t really tried it. Some people used them in a workshop I was teaching, but they weren’t as accurate as the graduated cylinders. I think the openings were too small.
To clean out the graduated cylinder, since I don’t have a sink, after pouring the glaze out I pour a tiny bit of water in, put my hand over the top and shake. Works really well to remove glaze residue and I can pour it back into the glaze.
I find I don’t normally have to remove water from a glaze once I got in the habit of measuring specific gravity. I’m usually adding water back in due to evaporation etc. But if I do have to remove water because I added too much, I’ll save it in a container until I inevitably need to add it back in 🙂
I can’t remember everything I said in the video. I should re-watch it! But there’s a good chance I’ve learned a few things since then. Thanks for commenting.
Sue
Oh my goodness. A Bedlington terrier. We used to have one. Love them.
Hi Sue,
Thank you so much for taking the time to explain how to find the Specific Gravity of a glaze. While up I have been making glazes at home for awhile, I have never measured the S.G. You have encouraged me to try this as glazing tends to be a bit of a crap shoot for me.
Thanks again so much! I’ll let you know how it goes with a couple of mo the when it’s time to glaze.
Thank you!
Hi from Albany, Western Australia, Sue. Love the tutorials and coming across incidentals I did not know. Extremely helpful. I have now calculated the SG on most of my home mixed glazes and I am looking forward to seeing how this changes the appearances of some I have been having difficulty with. Thank you!
You’re very welcome! Let me know how it goes.
Sue
I was taught to weigh a specific amount of water and the same amount of glaze and then calculate the specific gravity from those figures. Is that the same result as your method or is that giving me a measurement of something different?
Yes, that’s accurate. Except you don’t need to weigh the water each time because we know that water weighs 1 gram per millilitre. So if you weigh 300mL of water, it will weigh 300g. And if you weigh 300mL of glaze, you take the weight of the glaze and divide by 300 to get the specific gravity.
Sue
Hi Sue,
Thank you so much for this video, it’s nice to see it done in practice and make sure I’m doing it right!
I was using both a syringe and a graduated cylinder but was still having inconsistent results. I have a black matte glaze that needs to be 144 on the dot but has been coming out thin and shinier lately despite checking the specific gravity so wanted to check. I noticed when checking the accuracy of the markings that in both – when filled with water to the 100ml mark that it only weighed something like 96-97g. Does this matter and is this something that would affect my specific gravity results?
If you’re always using the same, slightly inaccurate measuring device, your results should be consistent. If you switch measuring devices, then your results would be inconsistent. So if you determined your glaze needs to be 144 using that same measuring device, you should be fine. But if someone else gave you that number as a recommendation using a different measuring device then your results wouldn’t be the same as theirs. Hope that makes sense. The number value is less important than consistency.
Thanks, Sue, I thought I was doing something horribly wrong (I started making glazes last month) when I kept having to add water and measure again and again and again.
Might an uneven application be the cause of a too-thick glaze? –and might this be remedied by thinning? Thanks again, looking forward to watching other tutorials of yours. -Pir
Sure, a too-thick glaze could lead to uneven application. Did you measure specific gravity the way I describe in this video? If so, what was the SG?
Sue, I really like your Cone 6 Eggshell Glaze.
What’s its specific gravity?
Thank you!
We keep it around 1.5
Our guild has recently embraced this method for our glazes. I understand the theory.
We use a 100 ml syringe. Makes the math easy. Just move the decimal point. We are just learning about what to expect for each glaze, generally aiming for between 1.45 and 1.50.
Some problems I have encountered…
One glaze had the consistency of pudding, yet the SG was 1.45. I added a significant amount of water, and the SG barely changed. It was still very thick, but I did a quick dip and the result was good.
I suspect minute differences in how much glaze goes into the syringe. I’m trying very hard to be consistent.
Mixing a new glaze…which was also very thick… the SG was also under 1.50. Added water, and again, very little change in the reading. This glaze has very inconsistent results, likely due to whether the person stirred it well or not.
I don’t use a mixer, but I do stir very well. (I’m too lazy to have to clean the drill mixer). I am wondering though, if the electric mixer adds air…which would change results, of course.
Thank you for sharing your knowledge! I sometimes feel like I am drinking from a fire hydrant, but it is starting to come together. You have a gift for explaining complex subjects so they are easy to follow.
Some glaze mixtures are more (shear thinning) thixotropic than others and this can affect the agglomeration of particles. With more agglomeration the more a clump will weigh in a given volume which can account for variations in SG. While bentonite can lend structure to a glaze mixture and hold particles within a framework this can lead to more uniformity if well mixed with high shear. Conversely there can be micro sized clumps of gel. CMC made as a thickener will actually act as a thinner and replace the attractive forces between particles set up by the bentonite structure. With CMC the glaze mixture flows more easily and the mixture becomes more uniform with a lower viscosity. This enables a more consistent filling of the syringe used for the SG measurement. The viscosity of the glaze mixture can be adjusted slightly with the concentration of CMC to make a creamy mixture with good retention on brush or slow settlement.