Crooked Lawyer Potter Posted February 28, 2022 Report Share Posted February 28, 2022 Sometimes it happens that I want to alter a glaze after it has been made up and the water added. Say, for instance, I have a bucket of glaze that I know is 15% silica and I want to experiment with it by adding another 5% silica to make it 20%. At the dry stage this is no problem, but now that it has the water in it I have to figure out how much of it's weight is water versus the formerly dry material. I'm thinking: Measure the specific gravity of the mixture. Lets assume it is 1.5. Measure the weight of the glaze (assume 15,000 oz). Doesn't this tell me that the (formerly) dry material in the bucket totals to 5,000 oz? Then, if I add another 250 oz of silica (i.e. 5% of 5,000), I would be at my goal of 20% silica. Am I thinking this thru correctly? Is there some other easier way to do this? Link to comment Share on other sites More sharing options...
Min Posted February 28, 2022 Report Share Posted February 28, 2022 1 hour ago, Crooked Lawyer Potter said: Is there some other easier way to do this? Yup. Bronginart's formula. Callie found a calculator for it a while back so you don't even need to do it by hand. https://wiki.glazy.org/t/brongniarts-formula/780 Link to comment Share on other sites More sharing options...
Jeff Longtin Posted March 1, 2022 Report Share Posted March 1, 2022 Years ago a photographer moved out of the warehouse, where I have my studio, and left behind a bunch of darkroom equipment. One thing he left behind were two plastic graduated cylinders. They've served me well for a number of uses. I use them to measure water, when mixing small batches of plaster, and I use them to measure liquid glazes. After I've mixed up a test batch of glaze, say 100 - 200 grams, I'll use the graduated cylinder to measure the liquid volume. I also have plastic buckets with volume measurements on the side. (Available at most hardware stores.) I use the buckets to volume measure a bucket of glaze. I'll then divide by the test glaze volume measurement, and I will have a pretty good idea how much glaze powder I have in any one bucket. For me 100 grams of glaze equals 4-5 oz of liquid glaze. (4 if it's a matte glaze, 5 if it's a clear glossy glaze.) This presumes you have a gram scale, in your studio, as well. Link to comment Share on other sites More sharing options...
Crooked Lawyer Potter Posted March 1, 2022 Author Report Share Posted March 1, 2022 Min Thank you so much for the reference to the Bronginart calculator. Just what I needed! But I confess I cant figure out why this formula would be true. Instead of my hypothesized 5,000 units of dry material the calculator says 8,125. Is there a simple common sense explanation of why my "logic" is wrong? If the slurry is 1.5 times the weight of water, and if the weight of the water is removed, why isnt the balance the weight of the dry material? Link to comment Share on other sites More sharing options...
PeterH Posted March 1, 2022 Report Share Posted March 1, 2022 45 minutes ago, Crooked Lawyer Potter said: Min Thank you so much for the reference to the Bronginart calculator. Just what I needed! But I confess I cant figure out why this formula would be true. Instead of my hypothesized 5,000 units of dry material the calculator says 8,125. Is there a simple common sense explanation of why my "logic" is wrong? If the slurry is 1.5 times the weight of water, and if the weight of the water is removed, why isnt the balance the weight of the dry material? Min beat me in replying to your original post, so I binned my reply. I post it now in partial answer to your new question. A surprisingly tricky problem. The easiest way is to use Brongniart’s Formulahttp://www.potteryatoldtoolijooaschool.com/brongniarts_formula_made_easy.pdf PS You still need to keep your wits about you. Was the original bucket 15% solids made up to unit volume of glaze, or 15% solids added to unit volume of water. PPS If you want a critique of your suggestion approach: Doesn't this tell me that the (formerly) dry material in the bucket totals to 5,000 oz? Only if the silica occupies zero volume (i.e has infinite density). Then, if I add another 250 oz of silica (i.e. 5% of 5,000), I would be at my goal of 20% silica. Is this a typo? If 5,000 of silica gives you a 15% glaze then 5,250 of silica won't give you a 20% glaze. Endnote: My brain hurts whenever I try to work this one out, and I have to revert to first principals rather than intuition. volume = volume-of-solids + volume-of-water weight = weight -of-solids + weight-of-water Link to comment Share on other sites More sharing options...
Min Posted March 1, 2022 Report Share Posted March 1, 2022 1 hour ago, Crooked Lawyer Potter said: If the slurry is 1.5 times the weight of water, and if the weight of the water is removed, why isnt the balance the weight of the dry material? It isn't just weight but the specific gravity (sg) of each of the materials added to the water. The calc I linked to from Glazy has a default sg for the materials set at 2.6 as an average of commonly used glaze materials. It isn't just the weight but volume plus specific gravity that are in the equations. I live in Canada where we have a mishmash of metric and Imperial measurements but I find it easier to use all metric when it comes to both liquid and dry materials, ie mls and grams (or litres and kilos). I find it much easier to think of terms of mls rather than oz, especially since Canada uses Imperial rather than US measurements for oz, pints, gallons. A real world example of this would be if you use a graduated cylinder to measure sg. For all intents and purposes insofar as glaze mixing goes 100ml of water will weigh 100 grams. If you use a graduated cylinder marked in oz 1 US fluid oz weighs 29.6 grams and an Imperial fluid oz weighs 28.4 grams. Much easier to work on the math using metric! Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted March 1, 2022 Report Share Posted March 1, 2022 +1 for everything Min said! For the love of all things holy, do not switch back and forth between Metric and Imperial when trying to figure out percentages of anything. There lies errors and madness. Plus, the conversions away from base 10 math make the relationships much harder to see. 4 hours ago, Crooked Lawyer Potter said: the slurry is 1.5 times the weight of water, and if the weight of the water is removed, why isnt the balance the weight of the dry material? The problem is, how much water vs solids are in that measurement? Brongniart’s helps extrapolate a weight measurement from a density measurement. When you’re measuring specific gravity, you’re taking a sample to see what the density of the suspension is, which is to say what the total weight of water plus solids are. Brongniart’s formula assumes that if your dry materials weigh a certain amount per gram, you can then extrapolate the solids to water ratio, which then lets you figure out the actual value of the solids and water. That brongniart’s calculator from Glazy is actually a bit of a fudge, because the formula assumes all dry glazes (which are a combination of materials that have different densities) are going to weigh the same. There is room to change that number in the calculator if you have more accurate info, but for the most part the number that’s auto filled is fine, unless you’re using a lot of really dense materials in unusual proportions. If you were in the unlikely position of using, say, a lead containing frit, the accuracy of that assigned value might be more of an issue. Link to comment Share on other sites More sharing options...
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