How to Measure Specific Gravity – Clay Week 2020
Here’s a live demo of how to measure the specific gravity of your glazes so you can have more consistent results.
Measuring specific gravity is a way to calculate and control the water content of your glazes. When the water content is consistent, application thickness will also be consistent, giving consistent results.
Thanks for watching!
*This video was first broadcast on Instagram Live*
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Thank you so much for all the information you’ve posted. I’ve just starting out after taking many community classes and have discovered how much I don’t know, especially when it comes to glazing and firing.
Please keep it coming!
Jill
Thanks so much Sue
That was wonderful
I finally get it ?
I’m in NSW Australia so a little awkward to join you live, however love to catch up on your vids afterwards
Thanks again
Sue, Thank you so much for sharing all you have learned about takIng some of the guess work out of glazing. A little overwhelming to think of converting my glazing process, but certainly worth starting to experiment with one or two glazes that are more difficult to nail down. As a person who loves math problems I was trying to come up with a way to calculate how much water to add to get the desired ratio. If I figure it out I’ll let you know!!! Lol. Thanks again for your generosity. Best regards, Robin
Hi Sue,
Well I did it! I figured out a mathematical way to know how much water to add to get a specific gravity. People who do not like math will probably opt for the trial and error method. In addition, the process would be a lot shorter if you knew how many grams and milliliters there were in your whole batch of glaze —- which would be easy to calculate when you were mixing a glaze from scratch. You could also mark a bucket in milliliter increments and you would need a scale that could measure the grams for a large amount of glaze or else weigh it in batches.
Well here goes.
How to calculate how much water to add to achieve a specific gravity of a glaze.
In the demonstration video, at one point you have 618 grams of glaze measuring 400 milliliters of liquid in the graduated cylinder. You divided to get the specific gravity. Mathematically this is expressed as:
618
400 = 1.545 specific gravity
So the question is, how much water do you need to add to the entire batch of glaze to bring the specific gravity to the desired result of 1.45?
As we don’t know how many grams or milliliters are in the entire batch of glaze (the bucket plus the beaker) the problem can be addressed in 2 steps, first finding out how much water you have to add to the beaker and then for the second step, how much water you need to add to the bucket.
So the equation for the first part is:
618_+__X_
400 + X = 1.45 (The target specific gravity)
represents the added water as one gram of water is one milliliter and adding water increases grams and milliliters the same amount
Solve for X
Multiply both sides by 400 + X
618 + X = 580 + 1.45X
Minus X from each side
618 = 580 + 1.45X
Minus 580 from each side
38 = .45X
Divide each side by .45
84.4 milliliters of water = X
PROOF
618_+_84.4 792.4
400 + 84.4 = 484.4 = 1.45
Second part
You need to know the weight in grams of the bucket of glaze (minus the bucket)
Let’s say the amount of glaze in the bucket is 3000 grams and given that the ratio of grams to milliliters should be the same for the bucket load of glaze as it is for the sample in the beaker we can express this ratio as
618 3000
400 = Y
To solve for Y, you cross multiply
618Y = 1200,000
Divide by 618
Y = 1941.75
PROOF
3000
1941.75 = 1.544 the same ratio as in the beaker
So what is the relationship between 3000 total grams and 618 grams?
3000
618 =4.85
If we multiply 4.85 by the additional water needed for the beaker we get the amount of water needed for the entire batch.
4.85 x 84.4 = 409.34 additional milliliters of water for the bucket
Add the water into the bucket ratio
3000 + 409.34 3409.34
1941.75 + 409.34 = 2351.09 = 1.45 specific gravity.
Best regards! Robin Bruck
P.S. Thanks for making me use my brain!!
I see some of the formulas shifted around,. I will email it to you.Robin
Wow, that is some impressive calculating there. I don’t have a scale large enough to weigh my full buckets of glaze so I couldn’t test this out. Have you tried it in practice?
What is the advantage of this method over using a hydrometer?
Hi Shirley, check out this article to find out why I don’t use a hydrometer https://suemcleodceramics.com/why-i-dont-use-a-hydrometer-to-measure-specific-gravity/
Sue