mr_glazy_man Posted February 8 Report Share Posted February 8 I don’t get pinholes in my later melting copper glazes, but I am getting them in my own earlier melting copper glazes. I need to be firing at exactly 1000°C, so I my idea is to incorporate a zinc to delay melting and give the copper an opportunity to finish off gassing. As a matter of curiosity, what temperature does copper typically finish its offgassing? Quote Link to comment Share on other sites More sharing options...
PeterH Posted February 8 Report Share Posted February 8 (edited) 8 hours ago, mr_glazy_man said: As a matter of curiosity, what temperature does copper typically finish its offgassing? While waiting for the experts to arrive ... I tried to find a decomposition vs temperature graph and went where the links led me. Digitalfire contains a large amount of somewhat confusing information under both copper carbonate and copper carbonate basic (which is what is sold). https://digitalfire.com/material/copper+carbonate https://digitalfire.com/material/copper+carbonate+basic The latter contains the most confusing decomposition graph I've seen. Not because of technical issues, just because its difficult to distinguish between the two lines presented. Picking up on the annotation "At 320C the remaining product is CO and all H2O and CO2 are driven off in one step" I found this simpler graph. https://www.researchgate.net/figure/TG-curve-of-copper-carbonate-NPs-sample-mass-30mg-heating-rate-10C-min-nitrogen_fig7_324064239 Which pretty unambiguously indicates that the copper carbonate decomposition/outgassing happens before 320C! Some of the items in the Digitalfire pages mentioned earlier may help make sense of the fact that glaze bubbling seems to be happening at temperature far above the decomposition point of copper carbonate, in particular https://digitalfire.com/picture/sagpedasyt The copper oxide recipe is not bubbling any less even though copper oxide does not gas. https://digitalfire.com/picture/1665 The top base glaze has just enough melt fluidity to produce a brilliant transparent (without colorant additions). However it does not have enough fluidity to pass the bubbles and heal over from the decomposition of this added copper carbonate! Why is lower glaze passing the bubbles? How can it melt better yet have 65% less boron? How can it not be crazing when the COE calculates to 7.7 (vs. 6.4)? First, it has 40% less Al2O3 and SiO2 (which normally stiffen the melt). Second, it has higher flux content that is more diversified (it adds two new ones: SrO, ZnO). That zinc is a key to why it is melting so well and why it starts melting later (enabling unimpeded gas escape until then). It also benefits from the mixed-oxide-effect, the diversity itself improves the melt. And the crazing? The ZnO obviously pushes the COE down disproportionately to its percentage. ... which gives a different rationale for adding zinc. Edited February 8 by PeterH Quote Link to comment Share on other sites More sharing options...
mr_glazy_man Posted February 8 Author Report Share Posted February 8 I’m so glad you posted those links, I’m hung up on exactly this as well. If decomposition happens at 320C, I would certainly not be having the issues I’m having because my high fluxed (read: high boron) glazes begin to melt at around 750-800C. The idea behind the zinc is to have them melt much later and more vigorously, at cone 05-06, but the exact amount depends on when the decomposition phase ends! Quote Link to comment Share on other sites More sharing options...
PeterH Posted February 8 Report Share Posted February 8 30 minutes ago, mr_glazy_man said: I’m so glad you posted those links, I’m hung up on exactly this as well. If decomposition happens at 320C, I would certainly not be having the issues I’m having because my high fluxed (read: high boron) glazes begin to melt at around 750-800C. I'm certainly out of my depth here. Before I'd read the Digitalfire entries I would have readily accepted your last sentence. Now I'd like to see some sort of "acid test", as I wouldn't have anticipated either of these results. https://digitalfire.com/picture/sagpedasyt Switching copper carbonate for copper oxide in a fluid glaze The copper oxide recipe is not bubbling any less even though copper oxide does not gas. https://digitalfire.com/picture/2387 This flow tester proves the colorant is not fluxing or bubbling the glaze Another observation: I was suspicious that the micro-bubbles in the glass matrix were coming from the copper carbonate gassing during firing. But not so, as you can see, the flow That is, testing the base glaze on its own or with the addition of non-outgassing copper oxide. Maybe I'll be less paranoid in a few days time ... Quote Link to comment Share on other sites More sharing options...
Min Posted February 8 Report Share Posted February 8 It's my understanding that copper carb releases carbon dioxide + water in the 300-320C range but then at 1050C it loses oxygen as it restructures (confirmed in this John Britt article). Since you are not firing as high as this it could continue to be an issue re bubbles. Re zinc, I think that if you are going to try it as an auxiliary flux I would be looking at using a frit that contains zinc rather than zinc oxide given that it is not an active flux at low temps below 1085C. (from Hamer's Dictionary of Materials and Techniques). PeterH and Kelly in AK 2 Quote Link to comment Share on other sites More sharing options...
mr_glazy_man Posted February 12 Author Report Share Posted February 12 Got it. I have FZ16 and FZ14 line blends to try and fix this - a zinc based frit will likely help. Will let you know how I go! Quote Link to comment Share on other sites More sharing options...
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