brookschneiderceramics Posted June 19, 2021 Report Share Posted June 19, 2021 Hi, I have just started mixing my own glazes. I'm using a recipe that has 1-2% cobalt carbonate and I am wondering if this is food safe. Thanks for your help! Link to comment Share on other sites More sharing options...
neilestrick Posted June 19, 2021 Report Share Posted June 19, 2021 Can you post the recipe? 1-2% seems like a lot, as cobalt is very powerful stuff. The makeup of the glaze will determine if it's food safe or not. Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted June 19, 2021 Report Share Posted June 19, 2021 What Neil said. The overall makeup of the glaze is more important than the presence of a given colourant, unless that colourant is something super unlikely like Uranium. If the glaze is durable, cobalt metal forms aren’t particularly soluble. We work with worse things. There was this thread a few weeks ago, where I did an ADHD research rabbit hole special. Tl;dr, You need to be more mindful about your own health and safety when you’re making the work than you do about the final glaze, unless the base glaze is really unstable. You’ll want to test it with a dishwasher tab. Link to comment Share on other sites More sharing options...
Hulk Posted June 19, 2021 Report Share Posted June 19, 2021 My fav blue, Lakeside Pottery Clear Blue, calls for 1.6% cobalt carbonate; next fav, Bill Van Gilder's "Variegated" calls for 1.5%, also some Copper Carbonate and Rutile; his "Teal Blue" - looks Teal to me - calls for 1% and also a smidge o' Chromium Oxide. All three seem very durable, sound glazes. I have made a commitment to clear or zirconium white-ish liner glaze (inside part of cups, mugs, bowls, etc.) for all food ware, which can be a bit more trouble; I'm stickin' with it. Link to comment Share on other sites More sharing options...
rox54 Posted June 20, 2021 Report Share Posted June 20, 2021 15 hours ago, Callie Beller Diesel said: You’ll want to test it with a dishwasher tab. Thank you everyone! I haven't heard about the dishwasher tablet. I heard about the lemon test. I will look that up. Thanks! Link to comment Share on other sites More sharing options...
Pres Posted June 20, 2021 Report Share Posted June 20, 2021 @rox54Lemon test is for acid resistance whereas dishwater tab is for alkali. Neither are exactly science, but if either causes the glaze to change, more testing is needed. @Hulk, I also like the glazes from Van Gilder. Got the book when it first came out, watched all of the series when they came on the DYI channel. I also use a liner glaze on anything functional for food or water. My recipe is from clear liner glaze that I added zircopax, tin, and rutile to. It is really more of a light butter cream on my SC630, and on the Hazelnut is much the same. Actually on the Hazelnut I use it all over, then spray glazes on as I do with the SC630. For spraying I use the variegated blue, Cream Rust, and the rutile green. I have had some issues with the Teal in the past, and have gotten back to using it as it works well now with the 630, not so well with the Hazelnut. . . .too muddy. best, Pres Link to comment Share on other sites More sharing options...
brookschneiderceramics Posted June 20, 2021 Author Report Share Posted June 20, 2021 20 hours ago, neilestrick said: Can you post the recipe? 1-2% seems like a lot, as cobalt is very powerful stuff. The makeup of the glaze will determine if it's food safe or not. It's a recipe from John Britt's book on high fire glazes. I'm very new to glaze making and i'm not too sure what makes a glaze stable or unstable. let me know if you have any advice! thanks so much Mamo Lavender Soda Feldspar 47 whiting 4 Silica 11 Kaolin 19 Dolomite 19 Cobalt Carbonate 1-2 Link to comment Share on other sites More sharing options...
Min Posted June 22, 2021 Report Share Posted June 22, 2021 On 6/20/2021 at 10:26 AM, brookschneiderceramics said: I'm very new to glaze making and i'm not too sure what makes a glaze stable or unstable. Having enough silica and alumina in the glaze, not overloading it with colouring oxides (transition metals), and firing the glaze to maturity (not underfiring) are three major things to look at. So, firing with witness cones to maturity solves one thing, amount of cobalt is the second, and looking at the recipe on the formula level address the third thing. There are "limit charts" to give an idea of the range of silica and alumina needed to make a durable glaze. If the glaze is durable there is less chance of leaching. A couple limit charts below. You can see for high fire it's recommend the silica be at least 3 (using the UK Traditional Limits) and 3.5 (using the Green & Cooper limits). I took your recipe and put it into Insight glaze calculation, screenshot below. Have a look at the silica level, it is coming in at 2.62 so it falls a fair bit below the "target" level of at least 3. Alumina looks okay. Just going by the numbers alone I wouldn't expect this recipe to be as durable as it could be. Link to comment Share on other sites More sharing options...
Min Posted June 22, 2021 Report Share Posted June 22, 2021 On 6/20/2021 at 7:03 AM, rox54 said: I haven't heard about the dishwasher tablet. I heard about the lemon test. I will look that up. Thanks! A few years ago myself and another potter were talking about Ron Roy's Licorice glaze with 2% cobalt carbonate. We both ran a base test (below) and put a piece of glazed ware in the dishwasher for 3 months. The base test did slightly remove some gloss whereas the actual running the piece through the dishwasher many times didn't. We came to the conclusion that the base test was too harsh a measure. Base test using soda ash to mimic dishwasher, use 50 grams of soda ash (sodium carbonate) to one litre of water, bring to a boil in a stainless steel pan and simmer for 6 hrs with the sample submerged. Compare this test piece with one that hasn't been and look for any difference in colour or sheen. Other thing you can do is send a sample to Brandywine Science Center, it's about $30- to test for one oxide. They will send you an email report showing how much of that oxide is leaching. Like others have said, looking at the recipe and formula can often give you a heads up on glazes likely to leach. Link to comment Share on other sites More sharing options...
brookschneiderceramics Posted June 23, 2021 Author Report Share Posted June 23, 2021 7 hours ago, Min said: Having enough silica and alumina in the glaze, not overloading it with colouring oxides (transition metals), and firing the glaze to maturity (not underfiring) are three major things to look at. So, firing with witness cones to maturity solves one thing, amount of cobalt is the second, and looking at the recipe on the formula level address the third thing. There are "limit charts" to give an idea of the range of silica and alumina needed to make a durable glaze. If the glaze is durable there is less chance of leaching. A couple limit charts below. You can see for high fire it's recommend the silica be at least 3 (using the UK Traditional Limits) and 3.5 (using the Green & Cooper limits). I took your recipe and put it into Insight glaze calculation, screenshot below. Have a look at the silica level, it is coming in at 2.62 so it falls a fair bit below the "target" level of at least 3. Alumina looks okay. Just going by the numbers alone I wouldn't expect this recipe to be as durable as it could be. wow thank you so much for this info! i appreciate it! its still kinda like greek to me but im learning! Link to comment Share on other sites More sharing options...
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