Algoessailing Posted April 2, 2021 Report Share Posted April 2, 2021 I know there are variables with glazes based on clays, kilns, fire schedule, etc, etc, but I did not expect to see variations within the same kiln. This is a photo of 5 pieces that came out of my kiln yesterday (a new glaze I am working with) The four small cups with letters are all different clays. When I first noticed the color variation I thought that the red clay may have something in it that made it more blue grey. However, the larger piece (upper leftmost) is the same porcelain as the cup labeled BU, and they are clearly not the same color. All five pieces were dipped in the same bucket, one after another, and fired in the same kiln load to cone 6. I don't remember if they were on the same shelf or not. All were bisque fired to ^04 and may or may not have been done in the same kiln load. I do know that I did 14 or 15 different variations of this glaze on test pieces and never saw the greenish hue that is present on the three cups. Any thoughts? Recipe: EPK 30 Wollastonite 30 Ball Clay 15 Frit 3124 15 Silica 10 Rutile 6 Volcanic Ash 5 Cobalt Carbonate 0.8 Green Nickel Oxide 0.8 Link to comment Share on other sites More sharing options...
Hulk Posted April 2, 2021 Report Share Posted April 2, 2021 Hi Sailing! Good question, noticed variations over same clay, particularly Rutile Green (from Bill Van Gilder's book) over white clay (Clay Planet Venus White). My thoughts on it include: slight variation in peak temp and cooling profile, perhaps heat work and cooling profile would be better, ahem, influence from neighboring ware - fuming, that is, and exposure to more/less oxygen. I'd like to say ware thickness and finish matters, however, am seeing marked colour variation among similarly built and burnished wares. My guess is that green/copper is unusually sensitive (tin red also seems rather sensitive, particularly to cobalt). I'm interested in what others may have to offer on the subject. Different colours, different clays, that's fairly typical, however, looks like some glazes, e.g. Teal Blue (also Bill Van Gilder's book) look about the same on different clays... Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted April 2, 2021 Report Share Posted April 2, 2021 Some glazes are definitely more sensitive than others. So I think the most accurate way to troubleshoot this glaze would be to compare as many apples to as many apples as we can. Your CK, CS and CC tests are obviously on 3 different clays, and any colour change could be most easily attributed to that. More on this in a minute though. When you look at the unmarked test and the BU so that you’re comparing the glaze on the same clay body, the sheen on them is markedly different. The unmarked test looks like it’s fully melted, even and has a soft sheen, while the BU seems to be somewhat less shiny, and looks like it’s got some bubbly/pinholey action going on. That leads me to think the BU might be slightly underfired, and was on a different shelf. What kind of kiln were you firing in, what was the cycle like and were temperatures verified on each shelf by cone packs? If a kiln isn’t full, is packed incorrectly or has aging elements, you can get as much as a cone difference from top to bottom. Interestingly, the BU, CS and CC tests seem to all have a similar sheen level, and are also closer to each other in tone. The unmarked and CK test are more similar to each other in both gloss and tone. I’d be very curious to know if those tests were on the same shelves as each other, and could help direct your next round of testing. Link to comment Share on other sites More sharing options...
neilestrick Posted April 2, 2021 Report Share Posted April 2, 2021 Looks like differences in temperature to me. The big pice is clearly melted more than the others. Put cones on each shelf next time your fire to see if the kiln is firing evenly. Link to comment Share on other sites More sharing options...
Hulk Posted April 3, 2021 Report Share Posted April 3, 2021 And here's a why your post caught my eye Sailing. I didn't notice earlier that your green glaze isn't copper. Any road, not remembering if these were on the same shelf. I like the left one better, will be watching for it in future... Link to comment Share on other sites More sharing options...
Algoessailing Posted April 3, 2021 Author Report Share Posted April 3, 2021 10 hours ago, Hulk said: And here's a why your post caught my eye Sailing. I didn't notice earlier that your green glaze isn't copper. Any road, not remembering if these were on the same shelf. I like the left one better, will be watching for it in future... Wow, thats really interesting. Other than the copper, how close is your recipe to mine? Link to comment Share on other sites More sharing options...
Algoessailing Posted April 3, 2021 Author Report Share Posted April 3, 2021 18 hours ago, Callie Beller Diesel said: Some glazes are definitely more sensitive than others. So I think the most accurate way to troubleshoot this glaze would be to compare as many apples to as many apples as we can. Your CK, CS and CC tests are obviously on 3 different clays, and any colour change could be most easily attributed to that. More on this in a minute though. When you look at the unmarked test and the BU so that you’re comparing the glaze on the same clay body, the sheen on them is markedly different. The unmarked test looks like it’s fully melted, even and has a soft sheen, while the BU seems to be somewhat less shiny, and looks like it’s got some bubbly/pinholey action going on. That leads me to think the BU might be slightly underfired, and was on a different shelf. What kind of kiln were you firing in, what was the cycle like and were temperatures verified on each shelf by cone packs? If a kiln isn’t full, is packed incorrectly or has aging elements, you can get as much as a cone difference from top to bottom. Interestingly, the BU, CS and CC tests seem to all have a similar sheen level, and are also closer to each other in tone. The unmarked and CK test are more similar to each other in both gloss and tone. I’d be very curious to know if those tests were on the same shelves as each other, and could help direct your next round of testing. Based on this, I would guess improper kiln loading. I know the large piece was on the bottom or middle shelf with taller pieces. The top shelf was probably 3-4 inches from the lid: I had a bunch of short pieces so I "fit" them in and likely the under fired pieces were there. The cone pack (top shelf) did not melt normally, only the ^5 bent slightly. The kiln is a paragon A88B (18x18inches) and the elements are fairly new so I didn't expect much variation in temp from shelf to shelf. In my mind a large kiln would have more hot and cold spots. However, the top element was at the same elevation as the shelf and was not able to heat fully, as indicated by the cone pack. I also had a few other test cups that behaved quite differently from previously fired test tiles. I'm now going to assume those too are under fired. Thank you for your diagnosis, it was very helpful. Link to comment Share on other sites More sharing options...
Algoessailing Posted April 3, 2021 Author Report Share Posted April 3, 2021 17 hours ago, neilestrick said: Looks like differences in temperature to me. The big pice is clearly melted more than the others. Put cones on each shelf next time your fire to see if the kiln is firing evenly. I usually only use one cone pack on the top shelf, I only have one usable peep hole in my kiln so I place it there. If the kiln is not firing evenly, what can be done? I very recently tested the elements and resistance was exact spec. Is your suggestion a for basic troubleshooting or is a cone pack on each shelf standard practice? Link to comment Share on other sites More sharing options...
Hulk Posted April 3, 2021 Report Share Posted April 3, 2021 Hi Sailing Here's the recipe (clip from Glazy, looks nicer - same numbers) Looking through my picture collection, almost all come out closer to the left teapot in colour. Link to comment Share on other sites More sharing options...
neilestrick Posted April 3, 2021 Report Share Posted April 3, 2021 2 hours ago, Algoessailing said: I usually only use one cone pack on the top shelf, I only have one usable peep hole in my kiln so I place it there. Put a cone on every shelf until you get it dialed in. You don't need to be able to see it during the firing, just to check when the firing is done so you can compare temp with results. If it's firing unevenly you'll have to adjust how you load it. Pack it tighter where it's hot, looser where it's cold. Link to comment Share on other sites More sharing options...
Algoessailing Posted April 13, 2021 Author Report Share Posted April 13, 2021 FWIW I did re-fire the three pieces to cone 6: Link to comment Share on other sites More sharing options...
neilestrick Posted April 13, 2021 Report Share Posted April 13, 2021 3 hours ago, Algoessailing said: FWIW I did re-fire the three pieces to cone 6: They look much better. Any variation in color I see there is what I would expect from the different clay bodies. Link to comment Share on other sites More sharing options...
Hulk Posted April 13, 2021 Report Share Posted April 13, 2021 Cool! I've a glaze load cooling now, will be watching the greens, and report back. Link to comment Share on other sites More sharing options...
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