HarryThePotter Posted May 12, 2016 Report Share Posted May 12, 2016 Hi guys!Recently I asked about a silicone ring inside a mug and received a lot of good feedback. As a result I decided to ditch the silicone and attach an aluminum oxide piece to the mug instead.I have attached a picture of the test results. It is so close!The alumina sticks well, but small cracks will be formed on the glazing. Do you know of any tricks to minimize cracking?The parts were fired to 1260C with high temp. glazing.-Harry Link to comment Share on other sites More sharing options...
glazenerd Posted May 12, 2016 Report Share Posted May 12, 2016 Harry: Only one thought comes to mind: the COE of the alumina is vastly different than the clay body and the glaze. Only trick I can possibly think of is an annealing cycle in hopes to "heal" the glaze. Slow cool at 200F from 1450F to 1100F. it "might" help. Sorry, a real oddity in the clay world: but I know about pushing the boundaries.. Nerd Link to comment Share on other sites More sharing options...
HarryThePotter Posted May 13, 2016 Author Report Share Posted May 13, 2016 Thanks Nerd. Can you tell me why you chose those numbers? Also, if the glaze is healed at 200F, then what happens when you later put 200F water inside the mug? -Harry Link to comment Share on other sites More sharing options...
Fred Sweet Posted May 13, 2016 Report Share Posted May 13, 2016 Harry- That's not what Nerd said. He said 200F/hour from 1450F to 1100F. Note: this is a cooling segment. Link to comment Share on other sites More sharing options...
HarryThePotter Posted May 13, 2016 Author Report Share Posted May 13, 2016 Ah, my bad. I'm currently doing the test with the temperature modification. Link to comment Share on other sites More sharing options...
glazenerd Posted May 13, 2016 Report Share Posted May 13, 2016 Fred: TY for clarifying: I miss the mark sometimes in communicating. Harry: The annealing temps that I suggested comes from the glass art industry: not the pottery world. "Dictionary of Glass materials and techniques" by Charles Bray (A&C Black-London) speaks about using an annealing cycle ( cooling segment in pottery) to help prevent glass fissures from forming when combining glass pieces with different expansion coefficients. Glass and glaze have the same composition/s: and I have used this technique for crystalline glaze which is notorious for crazing issues. At 1450F the piece is red, at 1250F it is pink, and at 1100F it has turned white. A critical period of cooling rather it is glass or glaze: which would only cause problems if the glaze/clay has an offset COE issue. I am not able to see the crack patterns: if they are concentric rings or evenly distributed cracks spaced closely together. Crazing would be concentric, elongated, and space further apart. Clay/glaze interface checking is a different rascal and would be seen as closely spaced crack patterns. You would certainly be susceptible to this issue as well: because the COE of the alumina oxide is that much different. The other issue you can only check by being near the kiln when it has cooled to around 450F to 250F. Most potters have heard the "ping" coming from the kiln at these temps: the last critical temp when COE issues make their presence known. You can expand the cooling segment from 1450F down to 1000F, just to make sure you extend past this critical period. This cooling segment is not required in most glaze firings: only when you know you have COE issues going in. Nerd Link to comment Share on other sites More sharing options...
glazenerd Posted May 14, 2016 Report Share Posted May 14, 2016 More thoughts: to give you an idea of what you are dealing with. I will post COE values of three individual products. Alumina Oxide:63.00 Frit 3110: 103.72 and NZ kaolin: 46.86. Very generalized, but gives you a sense of how your clay, glaze, and alumina oxide are creating surface tension/stress in relation to each other. The slower you can cool it at key temps: the better chance it has to make gradual adjustments in lieu of radical phase transitions. Trick 2: You do not have to fire every test piece to peak and back down to the suggested cooling segment in order to figure out the correct temps. In the glass arts: the method known as slumping will work just fine. In crystalline glaze it is known as a strike firing: often used to heal crazing issues. Take the cracked/crazed pieces and run them up to 1475F: then program the cooling segment/annealing cycle until you figure out the rate and range. Trick 3: Because you are dealing with an offset COE issue: it would also be advisable to cool the top and bottom of the piece at the same rate. The portion making contact with the shelf is cooling at a different rate because of the thermal conductivity of the shelf (thermal mass.) Corderite shelves in particular conduct and store a fair amount of heat. So I think in your case you need to get these pieces up off the shelf so the bottom cools at the same rate as the top. These are simple cone 10 clay slats that support the piece, and allows air to circulate underneath. You do however need to fire them to maturity before using so they do not move during the inversion phase. Nerd Link to comment Share on other sites More sharing options...
HarryThePotter Posted May 16, 2016 Author Report Share Posted May 16, 2016 Thanks Nerd! That was very informative. As to the results for firing with low temp. glazing (firing round 3): It cracked a lot more. Cracks also seem to originate from the corners of the alumina. For the next set I will definitely try trick 3 as well. I can imagine that it has a significant impact. It will also be a second firing to 1260C. On some pieces I will add alumina to the glazing for strengthening. On some I will add borax to adjust the CTE. As to your first thought of "the slower the better" - I've heard that glass may start crystallizing, which will cause cracking. -Harry Link to comment Share on other sites More sharing options...
PeterH Posted May 16, 2016 Report Share Posted May 16, 2016 Am I right in thinking that you are successfully joining alumina and porcelain, without cracks? If so, how feasible would it be to cover the alumina surface with a porcelain slip? This might reduce the glaze crazing problem, but obviously introduces the risk of the slip flaking off. PS Is the alumina essential? After all, some porcelains are used in hot situations: http://www.babaheatingedge.com/porcelain-heating-element.html Link to comment Share on other sites More sharing options...
glazenerd Posted May 16, 2016 Report Share Posted May 16, 2016 Harry: You are definitely wondering into new areas. So it will involve a learning curve. Will have to give some thought to the test result: "cracks more with a cooling segment." Interesting, and unexpected. Nerd Link to comment Share on other sites More sharing options...
HarryThePotter Posted June 1, 2016 Author Report Share Posted June 1, 2016 I got it to work after making my own porcelain slip with a higher COE. It seems porcelain COE is naturally very low, near 5, whilst alumina is at 8. Link to comment Share on other sites More sharing options...
glazenerd Posted June 1, 2016 Report Share Posted June 1, 2016 Bravo!! Ty for the follow up. Nerd Link to comment Share on other sites More sharing options...
Babs Posted June 2, 2016 Report Share Posted June 2, 2016 If cracking at corners...circular shape?? Link to comment Share on other sites More sharing options...
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