ImogenB Posted May 11, 2020 Report Share Posted May 11, 2020 I'm a new potter, and looking into making my own glaze. I'm concerned about how to make such a bright glaze on a stoneware body, most likely firing to cone 7/8 and that is this bright in colour AND most importantly food safe. I'm learning the chemistry behind it but ingredient suggestions might be helpful or just any help. Quote Link to comment Share on other sites More sharing options...
Min Posted May 11, 2020 Report Share Posted May 11, 2020 Hi Imogen and welcome to the forum. Cone 7/8 is an unusual cone to fire to, is this the range in which your clay matures or are you using one that is listed as having a broad firing range or is it a cone 10 clay? For functional work you want the clay to be vitrified. Looks like a cobalt blue satin or semi matte glaze. Using a clear or white gloss liner glaze is a good idea if you want to avoid the possibility of the cobalt leaching and cutlery marking with a matte glaze. You can do a couple home tests to rule out leaching glazes, there is also the option of having glazes lab tested. Glazy has quite a number of recipes, fill in your search terms on the left side of the screen. Cone 7/8 might not have many though. Having plenty of both silica and alumina in the glaze, firing it to maturity, not overloading most colouring oxides or fluxes and testing the glazes before putting them to use on functional pots are all important. Mastering Cone 6 Glazes is a great book for a fairly basic intro to making glazes and well worth reading if you are wishing to learn more about what makes a durable glaze. ImogenB 1 Quote Link to comment Share on other sites More sharing options...
Brandon Franks Posted May 11, 2020 Report Share Posted May 11, 2020 7 hours ago, ImogenB said: I'm a new potter, and looking into making my own glaze. I'm concerned about how to make such a bright glaze on a stoneware body, most likely firing to cone 7/8 and that is this bright in colour AND most importantly food safe. I'm learning the chemistry behind it but ingredient suggestions might be helpful or just any help. That is most likely a stain or a very saturated cobalt. Most saturated cobalts are dark blue, I have never seen such a light blue like that with cobalt. Look at these mason stains below. https://www.theceramicshop.com/product/8693/delft-mason-stain-6320-1-lb/ https://www.theceramicshop.com/product/8703/copen-6368-mason-stain-1-lb/ https://www.theceramicshop.com/product/8682/vivid-blue-6306-1-4lb/ Note: Stain are usually between 3-10% in glazes. If you want to make (say a 5000g batch) you are better off getting 1 lb of stain, many people (myself included) often get 1/4 lb and it's a few grams off, and there is not enough for a batch. ImogenB 1 Quote Link to comment Share on other sites More sharing options...
ImogenB Posted May 12, 2020 Author Report Share Posted May 12, 2020 14 hours ago, Min said: Hi Imogen and welcome to the forum. Cone 7/8 is an unusual cone to fire to, is this the range in which your clay matures or are you using one that is listed as having a broad firing range or is it a cone 10 clay? For functional work you want the clay to be vitrified. Looks like a cobalt blue satin or semi matte glaze. Using a clear or white gloss liner glaze is a good idea if you want to avoid the possibility of the cobalt leaching and cutlery marking with a matte glaze. You can do a couple home tests to rule out leaching glazes, there is also the option of having glazes lab tested. Glazy has quite a number of recipes, fill in your search terms on the left side of the screen. Cone 7/8 might not have many though. Having plenty of both silica and alumina in the glaze, firing it to maturity, not overloading most colouring oxides or fluxes and testing the glazes before putting them to use on functional pots are all important. Mastering Cone 6 Glazes is a great book for a fairly basic intro to making glazes and well worth reading if you are wishing to learn more about what makes a durable glaze. Hi Min, Thanks for your helpful response. I am firing to cone 8 to help the longevity of my kiln elements, my clay vitrifies from cone 6. The liner glaze is a good idea, but i am still worried i'd need excessive cobalt to achieve this and therefore would still be not food safe. I have just ordered the book, thank you for the suggestion. Quote Link to comment Share on other sites More sharing options...
ImogenB Posted May 12, 2020 Author Report Share Posted May 12, 2020 14 hours ago, Brandon Franks said: That is most likely a stain or a very saturated cobalt. Most saturated cobalts are dark blue, I have never seen such a light blue like that with cobalt. Look at these mason stains below. https://www.theceramicshop.com/product/8693/delft-mason-stain-6320-1-lb/ https://www.theceramicshop.com/product/8703/copen-6368-mason-stain-1-lb/ https://www.theceramicshop.com/product/8682/vivid-blue-6306-1-4lb/ Note: Stain are usually between 3-10% in glazes. If you want to make (say a 5000g batch) you are better off getting 1 lb of stain, many people (myself included) often get 1/4 lb and it's a few grams off, and there is not enough for a batch. Brandon, Thanks for your reply. I was wondering if it was stain. I'm yet to test one out, I shall get experimenting. This is the work of Helen Levi, I would think she tests her work to see if it's food safe, so i'm sure it can't be saturated cobalt as that wouldn't be safe. And thanks for the extra tip about amounts, i'll make sure to larger amount when ordering. Quote Link to comment Share on other sites More sharing options...
Brandon Franks Posted May 12, 2020 Report Share Posted May 12, 2020 (edited) 2 hours ago, ImogenB said: Brandon, Thanks for your reply. I was wondering if it was stain. I'm yet to test one out, I shall get experimenting. This is the work of Helen Levi, I would think she tests her work to see if it's food safe, so i'm sure it can't be saturated cobalt as that wouldn't be safe. And thanks for the extra tip about amounts, i'll make sure to larger amount when ordering. Here, I just came across this, looks nearly identical. I saw it and immediately thought of your posting. Give it a whirl and let us know how it comes out. https://glazy.org/recipes/2467 Also, You can probably squeeze this in a cone 8 load. If you want, I can make a test of it, Im getting rid of my last bit of HF clay, and don't have enough glaze for all the pieces Edited May 12, 2020 by Brandon Franks Quote Link to comment Share on other sites More sharing options...
Min Posted May 12, 2020 Report Share Posted May 12, 2020 (edited) 16 minutes ago, Brandon Franks said: I saw it and immediately thought of your posting. Give it a whirl and let us know how it comes out. https://glazy.org/recipes/2467 Would not recommend using a glaze with 40% barium carb, especially for functional work. https://digitalfire.com/4sight/hazards/ceramic_hazard_barium_in_materials_and_fired_glazes_26.html @ImogenB, not sure I'm following you when you say "I am firing to cone 8 to help the longevity of my kiln elements, my clay vitrifies from cone 6." I'm guessing this a clay that is advertised as having a wide firing range? Edited May 12, 2020 by Min Pres, Babs and Hulk 3 Quote Link to comment Share on other sites More sharing options...
Hulk Posted May 12, 2020 Report Share Posted May 12, 2020 Nice color! ...however, "Significant Flaw: potentially toxic" Barium, just not seeing a need, excepting for a smidge in scummy clays... Quote Link to comment Share on other sites More sharing options...
liambesaw Posted May 12, 2020 Report Share Posted May 12, 2020 19 minutes ago, Hulk said: Nice color! ...however, "Significant Flaw: potentially toxic" Barium, just not seeing a need, excepting for a smidge in scummy clays... It also says copper carbonate, I'm guessing to make barium copper silicate or "han blue"? Copper and barium that recipe would be very sensitive to any acid attack, probably straight tap water even Quote Link to comment Share on other sites More sharing options...
Brandon Franks Posted May 12, 2020 Report Share Posted May 12, 2020 56 minutes ago, Min said: Would not recommend using a glaze with 40% barium carb, especially for functional @ImogenBIf you use this you need a liner glaze. Sorry, I'm so used to using liners, I forgot to mention. Any clear cone 6-8 glaze would work. There are plenty in Britt's books. Quote Link to comment Share on other sites More sharing options...
liambesaw Posted May 12, 2020 Report Share Posted May 12, 2020 Here might be a place to start. Magnesium mattes with some calcium and cobalt can go pretty vivid indigo without having to get toxic. https://glazy.org/recipes/16778 ImogenB and Min 2 Quote Link to comment Share on other sites More sharing options...
ImogenB Posted May 13, 2020 Author Report Share Posted May 13, 2020 20 hours ago, Min said: Would not recommend using a glaze with 40% barium carb, especially for functional work. https://digitalfire.com/4sight/hazards/ceramic_hazard_barium_in_materials_and_fired_glazes_26.html @ImogenB, not sure I'm following you when you say "I am firing to cone 8 to help the longevity of my kiln elements, my clay vitrifies from cone 6." I'm guessing this a clay that is advertised as having a wide firing range? Yes I'm not sure that Glaze is suitable, I think i'll avoid using Barium. Yes it does have a broad firing range. I have an electric kiln, so it'd be an oxidise firing. I'm from the UK so using Fahrenheit isn't natural to me, so i'll use Celsius, but I usually fire to 1260oc. Quote Link to comment Share on other sites More sharing options...
ImogenB Posted May 13, 2020 Author Report Share Posted May 13, 2020 19 hours ago, liambesaw said: Here might be a place to start. Magnesium mattes with some calcium and cobalt can go pretty vivid indigo without having to get toxic. https://glazy.org/recipes/16778 Hi this looks great, thank you. I hadn't come across this recipe yet. Quote Link to comment Share on other sites More sharing options...
Min Posted May 13, 2020 Report Share Posted May 13, 2020 2 hours ago, ImogenB said: Yes it does have a broad firing range. I have an electric kiln, so it'd be an oxidise firing. I'm from the UK so using Fahrenheit isn't natural to me, so i'll use Celsius, but I usually fire to 1260oc. I thought that might be the case. Just a heads up about broad firing range claybodies, what is mature at the top of the firing range can't also be mature at the lower end of the range. A clay rated to go to cone 10 for example won't be mature at cone 6. I get that you want to extend the life of your elements but I'ld run some absorption tests on the clay to ensure that it won't leak at cone 7/8. If you haven't already done this there are directions on how to do this about 2/3rds the way down this page. Roberta12 1 Quote Link to comment Share on other sites More sharing options...
Babs Posted May 13, 2020 Report Share Posted May 13, 2020 (edited) There's "smash it before it multiplies"......nuff said Edited May 13, 2020 by Babs Rotate image Quote Link to comment Share on other sites More sharing options...
Babs Posted May 13, 2020 Report Share Posted May 13, 2020 (edited) Scroll down to vimeo.sorry re clunky..techidiot here https://www.dfb-keramiek.nl/2018/10/ceramics-fair-blues/?lang=en Edited May 13, 2020 by Babs Quote Link to comment Share on other sites More sharing options...
ImogenB Posted August 14, 2020 Author Report Share Posted August 14, 2020 Hi can anyone offer any further advice? I made this glaze and fired it to 1280oc. It is changing colour when i put in lemon over night to test for leaching, as you can see there are white speckles where the cobalt is no longer. The recipe is: Potash Feldspar 30g Quartz 36g Whiting 23g China Clay 11g + Cobalt Carbonate 1g Can anyone help to why this is still leaching? Quote Link to comment Share on other sites More sharing options...
liambesaw Posted August 14, 2020 Report Share Posted August 14, 2020 It has very low alumina to silica ratio Quote Link to comment Share on other sites More sharing options...
ImogenB Posted August 14, 2020 Author Report Share Posted August 14, 2020 4 minutes ago, liambesaw said: It has very low alumina to silica ratio How about adding 10% more china clay so the new ratios are Potash 27 quartz 33 whiting 21 china clay 19 do you think this would work ? Quote Link to comment Share on other sites More sharing options...
liambesaw Posted August 14, 2020 Report Share Posted August 14, 2020 Yes better Quote Link to comment Share on other sites More sharing options...
ImogenB Posted August 14, 2020 Author Report Share Posted August 14, 2020 10 minutes ago, liambesaw said: Yes better Would you otherwise think this glaze would be food safe? Quote Link to comment Share on other sites More sharing options...
liambesaw Posted August 14, 2020 Report Share Posted August 14, 2020 1 minute ago, ImogenB said: Would you otherwise think this glaze would be food safe? Yes. As long as it still melts with the added alumina. Quote Link to comment Share on other sites More sharing options...
ljescott Posted July 31, 2022 Report Share Posted July 31, 2022 (edited) Just fyi -- I'm not a potter but I have a set of dishes (plates, bowls, cups, saucers, and various serving pieces) that have an identical looking glaze as the Helen Levi set pictured at the top by the OP -- that bright, matte blue. I got them at a ceramics market in a town outside of Mexico City in 1989. I had some breakage tested when I got back to the US and was told they were not toxic, though the three or four times I did use them, I noticed that anything acidic (lemon juice or tomatoes) would spot (lighten? etch?) the finish. I put them away and have always meant to get them retested –– I'm not sure if staining necessarily means leaching. Until this post I've never seen any others like them. Does anyone know of a reliable lab in the Seattle area where I could get them tested? Edited July 31, 2022 by ljescott Quote Link to comment Share on other sites More sharing options...
PeterH Posted July 31, 2022 Report Share Posted July 31, 2022 Pure speculation, but uniformity of the glaze might indicate the use of a stain ... If the colour was due to an "inert" stain and the embedding glaze matrix did leach: what would leaching tests show? Presumably as the glaze matrix is leached stain particles would become free. But unless the stain itself was attacked by the leaching agent the offending elements would not go into solution, and would not be picked up by "wet" chemical tests on the liquid. Doubt that stain particles have been shown to be safe to ingest. PS The mugs and a Mason stain Quote Link to comment Share on other sites More sharing options...
Min Posted July 31, 2022 Report Share Posted July 31, 2022 15 hours ago, ljescott said: I noticed that anything acidic (lemon juice or tomatoes) would spot (lighten? etch?) the finish. I put them away and have always meant to get them retested –– I'm not sure if staining necessarily means leaching. Hi @ljescott and welcome to the forum. The fact that the glaze is changing when used with acidic foods is showing that it is leaching. Big question is what is it leaching. Personally I wouldn't use them. If you want to get one lab tested you would need to know what you want them tested for and for that we would just be guessing. Each oxide you get tested requires a payment. Ones that come to mind would be lead, barium and cobalt but could be any number of other oxides too. Then what do you do with that information as you already have evidence they are leaching. BSC does leach testing for pottery, link here if you are interested, they aren't in Seattle so you would need to mail in a sample pot, a mug would work. Quote Link to comment Share on other sites More sharing options...
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