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curt

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  1. Definitely keep posting the tiles - and your interpretation of what you are seeing. Very interesting and informative. Every Currie tile is a great learning opportunity, even if it is a glaze I don’t happen to be working on at the moment.
  2. A few more comments after a second careful look at these two copper/SiC tiles. I do like the second, third and fourth looks, because I am less distracted by colours and better able to hunt for the patterns and other fruit currie is offering up. First, suggest if possible you treat SiC as a flux in your calcs for this glaze. My take is that the second tile is overall noticeably more fluxed than the base tile. Specifically, compare cells 16 and 21 on both tiles. The SiC tile is much more crazed in those cells, making it more like its neighbours below. To me crazing is a sign of excessive flux (or, lol, not enough alumina and silica :-)) . In fact, overall the entire first column is much more fluxed on the SiC tile. Also, the whole right hand two columns of each tile compared, where we see the base tile very bubbly and crusty, and the same columns on the SiC tile smoothed out - pitted and bubbled yes (as you would expect with greater fluxing?) - but nonetheless flattened out, like the melt has been better or more complete anyway. On a "standard" currie tile with "standard" fluxes I think you would normally be looking for the "best" quality glass in cells 13, 14 and 15, and in cells 18, 19 and 20, where the alumina and silica are in about the right ratio, and with the right amount of flux to get the job done. However, in both your tiles, the good glass cells are much father over to the left as you have identified. This suggests to me that this glaze is very silica sensitive, and too much silica is just not a viable glaze, or more specifically, does not produce a good homogenous melt. Finally, very interesting to see where the red is in the SiC tile. At high flux levels, there simply is NO reduction red appearing. In fact, you could almost draw a line across the tile from the upper left to the lower right as the "reduction boundary", ie, combinations of silica and alumina that DO allow the reduction affect to occur, ie, where you can see red just starting to appear. Interestingly, at the bottom of this boundary, in the bottom row of the SiC tile, the red only appears in cell 34. Maybe it would have appeared in cell 35, but looks like this cell may not have melted properly. This all suggests to me that there is minimum silica and alumina that is needed to produced SiC copper red? Also, that this glaze is VERY sensitive to silica....
  3. Maybe what this guy suggested? Just substitute the word “copper” for “iron”? Lol. Maybe. However, looks like those right hand side SiC cells are kind of pittted, no?
  4. Great work, Joseph, very interesting. And some very rich glaze surfaces there. Thanks for sharing. A few thoughts: First, would you be able to share photos of one or two of the actual currie grids you did for this? Also, in the side by side samples, are you saying these two glazes are identical except for one has silicon carbide and one does not? Regarding the bubbles issue, since you are starting to get a feel for the actual critical alumina and silica levels in your testing, I would be interested in your thought/comments on a thread started by High Bridge called "Bubble, Bubble, Toil and Trouble..." about a year ago (? you may remember it) where we had an extensive discussion about silica and bubbles based on some tests High Bridge shared. Do your recent results shed any more light on the bubbles issue? Are they in line with the outcomes in Joel's tests? Seems like you are highlighting .25 and 2.5 molar on alumina and silica as an important threshold. Could it be that the "limits" are more context specific to certain industrial conditions (ie, specific glaze mixtures, specific clay bodies, specific firing procedures) than we are aware, and hence not necessarily applicable to many glazes we look at? I guess for a decorative glaze like yours maybe the limits are kind of irrelevant anyway. I had a quick look for some papers on the silicon carbide iron eutectic point. Seems that molten iron (and also cobalt and nickel) is an excellent solvent for silicon carbide, starting at temperatures well below the stoneware maturing range. Maybe the iron-heavy glaze mixture is simply decomposing the silicon carbide early and often, preventing it from delivering the usual cratering? And, giving it that nice smooth satin matte texture. Milk frother seems to work excellent!
  5. Interesting, thanks for sharing. Seems like your material is quite refractory, no? have you considered calcining it first to burn out the organics? Might make it easier to use.
  6. Wood firing conversations?

    Yep, wood firer here, in season anyway... and it’s good that you asked because I also have some questions that I would like to post in due course.
  7. How to Replace Ferro Frit 3249

    If the COE of the clay is too much lower than the clay then maybe shivering will be an issue? Would be interested to know if you end up having this problem. I think Min has suggested a good strategy for attacking the adjustment in stages.
  8. I would not get too hung up on one particular frit. If you don't like 3134, there are hundreds of different frits out there and probably a few others with not dissimilar chemistry would do the job for you if you wanted to play around with it. All it really means is that you will need to tweak the chemistry of the whole glaze to accommodate the particular frit you CAN get your hands on. You could even go upstream to boron itself (rather than boron packaged up in a frit) if you can get that (borax? boric acid?, etc.). But like everything boron has its trade-offs. It is all a game of compromise.... I think you are right to look at the claybody. I am not quite the purist regarding only potassium and sodium as some others - they are not the only fluxes that can aid in glass formation, Limoge being a perfect example. But there can be little doubt that they will beef up the COE of a clay body. Particle size aside, the high shrinkage of most commercial porcelains is directly related to these two fluxes, potassium and sodium.
  9. This looks like a classic case of why it is sometimes easier to adjust your clay body than your glaze, and it seems that after a long journey that is about where you are at in your thinking. But...a few comments on your glaze: Your adjusted glaze as it stands does not look viable to me, especially not at cone 5. Your recipe now has a much larger than usual proportion of very refractory ingredients (silica and alumina), and very little in the way of "power fluxes" such as potassium and sodium to melt those refractory ingredients. I know you have dumped most of those fluxes for COE reasons, but you can only take that process so far before, as you suspect, the glaze is totally out of wack and I suspect it will not melt. I think the trick with these fluxes is to lower them some, but not too much. Only testing will tell how much. You have lots of calcium, the "bread and butter" flux of stoneware glaze temperatures, but at midfire temps (cone 6 and below), your calcium is not going to do much melting work. Calcium has a eutectic with silica and alumina (eutectic = the lowest temperature all three will melt at together if they are in just the right proportions to each other) at somewhere around 1200 Celsius, but this will barely be getting going just as you are hitting your top temp for a cone 5 firing. So calcium is kind of sitting on the sidelines in this glaze. Magnesium (which is even more refractory as a flux than calcium), is also not going to be doing much to help melt things at these temperatures I suspect. So even though your Silica:Alumina ratio appears that it would give you a super glossy glaze, due to the very small amount of low temperature fluxes I think you would find this glaze dry and unsatisfying due to all that silica, and probably not even properly melted. Might want to try a line blend with ever increasing silica in this glaze to see first. The one thing that might save this glaze if you are firing in reduction is all that iron. It is a very active flux in reduction, and it might just come to the rescue in the glaze melt. But that is a long shot in my view, never a good idea to depend to much on reducing iron as a flux driver, not least of all because reduction can be uneven about the kiln. All this is one reason glazes start to lean heavily on frits, and particularly the hefty amount of boron they contain, once one leaves the realm of stoneware temperatures. That is something I see you have not tried (?), ie, upping the amount of (relatively lower expansion) frit instead of depending on increasing silica to crazy levels. Frit 3124 is billed as a low melter, but I think you might want to sub in 3134 because it is an even better melter, mostly because it has no alumina. Anyway, just a thought about another way to go with adjusting your glaze.
  10. Home Made Kiln Controller

    That table in your link enables the conversion of millivolts to degrees Celsius. If you don’t happen to have a proper thermometer to hook up to your thermocouple, you can hook up a multimeter instead and you get a reading in millivolts which, using this table, can be converted to an equivalent in degrees Celsius. Or maybe you knew that?... anyway, very handy if you have a spare multimeter lying around. Your table is for an S type thermocouple, but as you may have seen there are equivalent tables for all other types of thermocouples as well.
  11. Home Made Kiln Controller

    One useful feature for the software would be to have one screen option that simply displayed the temperature(s) as big (giant) numbers and nothing else on the screen. One big number for each thermocouple channel, that the number could be seen and read easily in all kinds of ambient light from 20 or 30 feet away. This would make it easy to know the temperature from anywhere in the general vicinity of the kiln (say, down on hands and knees squinting into a porthole) without having to get up and walk over to a pc or laptop and search on a screen. I am looking for something like this right now to display thermocouple temps for a wood fired kiln and have had no luck so far finding it. If anyone has any ideas?....
  12. Search this forum for threads on Ian Currie test tiles. I think this is very similar to what Dave is talking about. It is a good way to examine the effects of systematic changes in silica and alumina on a glaze.
  13. I was in Berlin a couple of years ago and found a small studio in Kreuzberg around the corner from my apartment making hand-thrown lamps out of Limoges. I spoke to the woman who ran it and she gave me contacts for a couple of other potters in Berlin but I ran out of time. I will have a look for her card. Very interested in what you discover as I am not sure if we have many German potters on the forums?
  14. LT thanks for this amazing reading list. I have spent some time with Shelby, very good value. Will trawl through the rest in days ahead. The only thing that bothers me a little about trying to apply lessons from the glass guys is that they do not try to make their glass batches on top of a ceramic mass underneath which they are trying to mature at the same time! To your point about igneous petrology, I found a paper recently on magma recently that seemed like they could have been talking about glaze! A silicate melt which heats and cools and gets affected by all sorts of alkalis, metals, etc. along the way. Surprising parallels.
  15. Agreed. It was not so much a conclusion, but rather more of an idea about a path for further exploration to help solve your mystery. My take on glaze eutectics is that the more oxides you have, the more complex eutectics have the opportunity to form, and the lower the temperature at which everything melts. Thinking more about it, I agree with you that the boron could be the catalyst. Boron starts melting at around 840 C and is a glass by 950 C, so (depending on how much boron you use) this could be getting the overall melt going way earlier than typical midfire temperatures. I wonder if this also means that oil spots could appear in a reducing environment if the temperature at twhich the melt really got going was lower than when the iron products in question started to decompose?
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