Jamie page Posted December 18, 2014 Report Share Posted December 18, 2014 Newly here Have started drinking coffee from glossy black mug for a few days and the inside of mug has turned dark blue. Is this normal or is it a mistake from my glaze? The recipe came from ceramicartsdaily.org. I am new to the pottery world and would never want to harm anyone drinking from my mugs. Thanks for any replies Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted December 18, 2014 Report Share Posted December 18, 2014 Welcome to the forum! Also, congratulations on discovering the leach test under your own power . Not the most auspicious way to discover it, but better sooner than later. A glaze that changes colour like that when exposed to a mild acid like coffee, tea or vinegar is soluble, and shouldn't be used on the inside of a functional vessel. This does happen with shiny glazes, not just matte surfaces. The likelihood of you acutely poisoning anyone healthy from drinking out of such a mug is pretty low, but materials that dissolve out of a glaze might be detrimental to someone with a compromised immune system, or who is on certain medications. Probably not good for anyone in the long term. Link to comment Share on other sites More sharing options...
Jamie page Posted December 18, 2014 Author Report Share Posted December 18, 2014 Great Chantay, I meant only that some details are obvious to some and not others, I was really glad when my eye was drawn to this aspect of my pots. In this forum, seems we are open to any hints so I passed it on! Hope no offense was taken On the unglazed areas of some of my pots I recently tried the soda wash that Chris Campbell kindly described to us. it is worth a try , doesn't alter the colour but takes it from clay to a satinny feel which I kinda like. Link to comment Share on other sites More sharing options...
Jamie page Posted December 18, 2014 Author Report Share Posted December 18, 2014 Thank you for your information Link to comment Share on other sites More sharing options...
Babs Posted December 18, 2014 Report Share Posted December 18, 2014 Thanks, the recipe for the s. wash is not at the fore of my brain, but you can do a search on this site and you'll find it. Do you know what was in your black Glaze? Link to comment Share on other sites More sharing options...
Jamie page Posted December 19, 2014 Author Report Share Posted December 19, 2014 Gerstley borate Whiting Custer feldspar Epk Colbslt oxide Copper carbonate Manganese dioxide Link to comment Share on other sites More sharing options...
Babs Posted December 19, 2014 Report Share Posted December 19, 2014 Nice bunch! %manganese? Check post on Manganese dioxide. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted December 19, 2014 Report Share Posted December 19, 2014 Please give us the recipe with percentages / amounts. best, .................john Link to comment Share on other sites More sharing options...
Jamie page Posted December 19, 2014 Author Report Share Posted December 19, 2014 Gerstley borate 10.7 Whiting. 5.5 Custer feldspar 78.9 Epk. 4.9 Add Cobalt oxide2.1 Copper carbonate. 4.1 Manganese dioxide 4.1 Link to comment Share on other sites More sharing options...
Jamie page Posted December 19, 2014 Author Report Share Posted December 19, 2014 I have just read the suggested post and wonder if there is another commonly used chemical I should be made aware of? Link to comment Share on other sites More sharing options...
Babs Posted December 19, 2014 Report Share Posted December 19, 2014 Really importatn that you know what the properties of the chemicals you are dealing with , both for your safety as a potter handling and firing chemicals, and for the eventual users of your pots. Some one on this forum would be able to advise on a suitable source of info for you. Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted December 19, 2014 Report Share Posted December 19, 2014 What cone does this fire to? 6 or 10? It seems like the alumina might be low, given your low percentage of epk, but someone with a copy of some glaze software probably could say better. And your colourant percentages seem high, as well. Most of those are quite potent. Link to comment Share on other sites More sharing options...
High Bridge Pottery Posted December 19, 2014 Report Share Posted December 19, 2014 http://digitalfire.com/4sight/education/index.html is a good place to start researching your oxides. Link to comment Share on other sites More sharing options...
bciskepottery Posted December 20, 2014 Report Share Posted December 20, 2014 The recipe came from ceramicartsdaily.org. Could you post a link to the page on ceramicsartsdaily.org where the glaze recipe was posted? Link to comment Share on other sites More sharing options...
Guest JBaymore Posted December 20, 2014 Report Share Posted December 20, 2014 OK without doing ANY formal glaze calculation in Insight...... let's look at that colorant supply. That's a total of 10.3 percent of metallic oxides. There are few to no glazes that can hold that kind of saturation of such coloring oxides in solution as the glaze melt cools. Just ain't gonna' happen. So some of the coloring oxides are going to precipitate out on the surface of the glaze. Any oxides that are in this condition are almost certainly subject to easy leaching into strong-ish acids and bases. Then looking at WHICH oxides these are. Copper is notoriously hard to keep in solution in glazes. And at 4% that oxide alone is flirting with the top amount that can even closely be stable. Then there is 4% of cobalt OXIDE. That alone is enough to make the glaze almost black and be supersaturated. The manganese is there just for good measure to totally make this absurdly oversupplied with colorants. Then take into account that this looks like a Cone 6 recipe. At Cone 6 a significant amount of boric oxide glass is introduced to lower the melting point. Boric oxide glass is softer than silica glass... so this is a less durable base than if it were at Cone 9-10 or higher. SO the glass has a lower ability to hold the oxides that are still IN solution also. (note... yes adding B2O3 in very precise proportions to the SiO2 can harden borosilicate glasses... but just looking at the recipe, I don't thik that is the case here.) If this is getting fired atr cone 9-10....... worse... don't need that boron. NOT a glaze for food bearing wares. Period. best, .......................john Link to comment Share on other sites More sharing options...
Guest JBaymore Posted December 20, 2014 Report Share Posted December 20, 2014 Just ran it in Insight. Also it is WAY oversupplied with KNaO for both cone 6 and cone 9. (Other oxides are within limits). Those are not helping with the leaching stability either. best, ...................john Link to comment Share on other sites More sharing options...
Jamie page Posted December 20, 2014 Author Report Share Posted December 20, 2014 Article- Glazes for oxidation firing in electric kilns...by Jayne Shatz. This article was a part of "Techniques and Tips for electric kilns" Supported by Skutt on the ceramicartsdaily.org 2014 Link to comment Share on other sites More sharing options...
Babs Posted December 20, 2014 Report Share Posted December 20, 2014 Thanks John, wish I could retain at leasst half of what you write. Babs Link to comment Share on other sites More sharing options...
Marcia Selsor Posted December 21, 2014 Report Share Posted December 21, 2014 Good work John. I remember when that article came out. I felt that some of the glazes had high oxide content as the Iron red has 15% red iron oxide. Thanks for checking those. Maybe someone should be doing this for CAD before posting. Marcia Link to comment Share on other sites More sharing options...
Guest JBaymore Posted December 21, 2014 Report Share Posted December 21, 2014 OK... venturing into "dangerous grounds" here................ going off the "politically correct" spectrum........ This discussion above cuts to the core of one of the things I always mention in my ceramic chemistry classes. Which I think is VERY important for students to really understand. It gets to core ideas of "how the world works". In general, how someone gets to be a "famous ceramic artist"........ is from the visually apparent aesthetic qualities of their work, the technical execution of the pieces (from a forming standpoint), and is often combined with the way they present themselves in public settings (exuding a professional feeling and being nice people). Maybe add in a dose of "right place at the right time" serendipity, and also a bit of a "who you know" factor. What this "fame" does NOT necessarily tell you is their depth of understanding of the technical sides of the process. For all anyone knows... they flunked or got a D in their ceramic materials / ceramic chemistry / kiln design-building classes (if they ever had that kind of training). They may have mastered the aspects of the process to get the visual result THEY use...... but that might be the real limit of their technical expertise. They might even have had training that was not all that accurate, and are sharing that stuff once again, spreading the misinformation. So just because "Famous Ceramist" shares some glaze recipe or some ideas on the technical side of things........ that does not make it automatically "truth". There are things I've seen printed in studio ceramics oriented books that fly in the face of basic science and engineering principles. Sometimes the information presented by "Famous Ceramist" is taken out of context, and then transferred to others incorrectly. For example, a glaze used solely for sculptural work gets shared... and then a participant in a workshop comes back and shares it with their functional potter table-ware producing friends. Suddenly it shows up on tableware. Then that tableware potter (maybe who has little technical knowledge) becomes "famous"... and that glaze then goes on to be shared as a tableware glaze. And so on. Vet your sources folks, vet your sources. Sometime the emperor has no technical clothes. best, ................john PS: Luckily, most of what we do is not "death incarnate". But having a good grasp of some basics is important. Link to comment Share on other sites More sharing options...
Benzine Posted December 21, 2014 Report Share Posted December 21, 2014 Preach on John! I'll out myself as a propagator of false information, not knowingly of course. For years, I told students that air pockets caused explosions. Why? Because that's what I was taught, and had that belief reaffirmed many times over. I also was always under the belief that glazed low fire wares were always safe for use. I didn't know about crazing (other than it looked cool for Raku effects), and definitely hadn't heard of leaching. Don't get me wrong, I wasn't promoting the use of "decorative" glazes for food use, like "Hey kids, that glaze looks cool, so go ahead an use it on the inside!" The companies are nice enough to put some guidelines on the bottles. Sure, my misinformation wasn't horribly detrimental to my students. Removing air pockets, is still good practice, and I doubt any of them will be poisoned from food safe commercial glazes. But my time on these boards have definitely changed the way I present information. I only wish I could get a credit towards my license renewal for our discussions here. Honestly, I've learned more, that I can apply to my teaching here, than I have in the classes that I actually can count... The internet as a tool for sharing of information and ideas; who would have thought? Link to comment Share on other sites More sharing options...
Jamie page Posted December 21, 2014 Author Report Share Posted December 21, 2014 My husband is a retired physician and has a good working knowledge of chemistry and is learning glaze chemistry. He has started mixing my glaze for me. Any recommendation for glaze calculation program? Jamie Link to comment Share on other sites More sharing options...
Guest JBaymore Posted December 21, 2014 Report Share Posted December 21, 2014 Jamie, Insight Level II from Digitalfire.com. I have been using that since god was a kid. The program has all the capabilities you will EVER need. And with a science type background, your husband will likely enjoy the subscription access database / website that comes with the Level II version. best, ...............john Link to comment Share on other sites More sharing options...
Babs Posted December 21, 2014 Report Share Posted December 21, 2014 Now my xmas wish is to be reborn at the age and financial enough to attend J Baymore's classes, just next door to where I live.. Oh I would travel to do this. However, this forum is fantastic in the way the knowledge is freely shared. Thanks. Babs Link to comment Share on other sites More sharing options...
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