Some results
Above are the results of the firing (following on from my last post). I was predominantly making a batch of dark blue beads as I knew they would not be affected by 'the flash'. In this firing I did a couple of test wires to see if I could subdue 'the flash' on the lighter copper, turquoise colours.
Following Anitra's advice I painted some of the wires with kiln wash and I left some of the wires bare. I did, however, clean down all the wires first - grinding off any residue from past firings. I also used a variety of gauges of wire from .7mm to 2mm. After painting the wires I left them to dry thoroughly and actually left the beads to dry out a little as well before carefully threading them onto the coated wires. They went on smoothly enough with little disruption to the kiln wash on the painted wires. There seemed to be no indication of the 'the flash' during making or during drying - even on the wires that were uncoated.
BUT when I opened the kiln today there it was! It had appeared on the gaps of the wires between the beads - on both the wires painted with the kiln wash and the plain wires. I was hoping it would have been contained to these areas but on closer inspection I can see it still migrated to the surface of some of the beads.
So I am due to make a large batch of pale colours and of course am concerned as to how to beat this. Anitra, you suggested coating the wires in a wax of some sort? Do you mind explaining that process please? My mind is telling me that the wax will just melt....... Also I have read that another ceramist having this problem coated her wires with enamel but again I have no idea what that would entail.
Is there another kind of wire that could be used in place of a nickel based wire? Perhaps stainless steel - is it ok to fire this in an electric kiln? I wonder if it is possible to source thin 'ceramic' wires of some description as it seems to me that it is the metal that is the key to encouraging the growth of 'the flash'.
posted by kriket @ 12:18 AM
5 Comments:
We have Lots of very different issues to address here on the eve of my hiatus from the Internet for two weeks. But I'll try to head you off at the pass here. The stainless steel wire has high chrome content, 10% or more depending on the quality.
· Using stainless steel will probably multiply your problem by a factor or two. The iron component of stainless steel wire contains a host of undesirable properties too. But by all means try it just to prove it won't work. My opinion, is you can learn more from a solid failure than a marginal success.
· What are you using for your green colorant? Does it contain chrome?
· Enamel!!?
· Kiln wash for Egyptian paste is a one-to-one ratio of aluminum hydrate to kaolin, that is by weight rather than volume. One omits the silica component of shelf primer. We need to be certain that we are on the same page.
· As for waxing, from what you described waxing isn't going to help. There are two types of wax hot wax and wax resist. For the hot wax, you would just dip it. For wax resist/emulsion; you painted on; normally one layer will suffice. Yes, wax burns off in the initial stages of the firing. Because we determined that that the flashing is happening in the kiln and not in the drying process, waxing is totally unnecessary.
· If you look back in this blog you'll find a video on making a bead tray/island. It renders down to a porcelain mandrel in the bead hole. Brush a couple of layers of kiln wash on the bead tray; your recipes forms thick glaze, you don't want to be drilling porcelain out from your holes. Therefore, the “island” will work for your 2 mm bead holes. There is serious doubt it will work for .7 mm holes, unless you are really perfect at tapering porcelain.
These are a few thoughts I had for just now. When I'm back online, I would have had time to ponder over it. Perhaps I'll come up with a solution. Did you run the experiment to see that the wire is the culprit? I'd like to see image of that BTW. Let's confirm we've identified the problem correctly.
Kriket - this is all most interesting to me. I'm hoping to learn how to make accurate replicas of faience beads from Egyptian and Roman times. I'm not a ceramicist, but I am also keen to experiment some day with porcelain, which of course needs even higher temperatures... So, what would you use for firing porcelain beads? Would you expect to see the same problems with nichrome that you're experiencing now? Or maybe it is to do with the sodium salts in the paste and so would not be an issue with clays.
I'm not a chemist any more than I am a ceramicist, but chromium and sodium do form a compound with oxygen - Na2CrO4 - known under several different names, most commonly Sodium Chromate. It is bright yellow in colour and is used as a pigment in certain dyes. Do you think you might be producing that? It is quite a nasty toxic (possibly carcinogenic) compound - though I imagine in tiny quantities encased in glaze it couldn't pose any significant risk... It might, in any case, be worth talking to a chemist to see if they could explain what's causing it and how you could prevent it.
Hi Ajda, sorry for the slow reply but I have been away for weeks. Hmm interesting - I guess it could be sodium chromate that is developing. I do not use straight chrome in my mixes (but I do uses some stains and there is possibly chrome in those). I am concerned of toxicity levels and endeavour to take care in my handling of powders and oxides. I wonder if it is more toxic during firing..... I did speak to someone in the industry who was surprised that I was experiencing this problem at such low temps and he suggested firing with the kiln bungs open for the entire firing. I have only just made a batch of beads to test fire on the weekend - so will leave the bungs out and see what happens.
In response to your qstn, I have never fired porcelain so do not know if you will experience these problems - but I do remember reading online somewhere about an artist making porcelain beads and having this problem with the white beads. So I guess there is a possibility. Good luck and let us know.
Can you fire egyptian paste in the same kiln used for clay pieces that are finished with commercial glazes?
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