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Post by Arnie Benton on Dec 13, 2015 10:52:24 GMT -6
Good idea, Tom -
I tried the tap test on the few bisque pieces I have left - one of them was a clunker - and it has a visible crack - hopefully this test will work with less visible defects - wouldn't you know, the defective piece is 17 inch diameter. But saves a lot of wasted effort.
Arnie
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Post by jfox on Dec 13, 2015 12:14:42 GMT -6
i think you should start testing some other clays or make some adjustments to the one you are using. i was having a problem with teapots shattering when filled with boiling water i added some 100 mesh mullite to the formula seems to be working, you might test the adsorption of the body it might be overfired
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Post by mohawkpiper on Dec 14, 2015 3:05:41 GMT -6
maybe it's glaze thickness that's the most important variable to manipulate. I have felt like this is the case for a while now. Though glaze thickness can be manipulated through many more ways on vertical compared to on flat. On flat the applied thickness correlates much more with the thickness of the glaze at time of growth. Much much more so than on vertical, where clay thickness, peak temp and melt of glaze (chemistry) play just as equal a part as do applied thickness to get the right glaze thickness at time of growth. Of course zinc amount also matters almost just as much as thickness of glaze. So Evan - I could send you on a 'wild goose chase' with glaze additions and subtractions, but none of them ever worked for me - so try putting on less glaze first - Arnie Keep in mind a glaze can be altered to work at various thicknesses, and what may be a lot of glaze for one person may not be that much glaze for another. More crystals from my experience generally says glaze is too thin. Or it has too much zinc. Or it isn't melting enough. (that is the order I attack my problems in haha.) Jerry - forgot to say - I try to stick with the golden ratio with the lines of those platters. Though I eyeball it so it's never really all there properly. The ones that come out more attractive generally have been closer to that ratio though. Arnie - awesome it is working better for you! G
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Post by jerrysawitz on Dec 14, 2015 11:15:03 GMT -6
greg, yeah I can see that. however, amount of crystals are the wild card there. I think you are on to something really good, combining chattering with glaze though. Seems like you stained it too, perhaps a light iron wash? But it reminds me of the ocean and shoreline. Golden sand and blue water.
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Post by evan cornish-keefe on Dec 14, 2015 11:59:08 GMT -6
Hey Arnie, I agree with Jim that you should test your clay for absorption, and not bother trying to get a glaze to fit a clay-body that's dunting. This is Stull's map of silica and alumina, the cross hatched area shows where a glaze will craze. Ray Stull tested this on a porous underfired clay in order to accentuate craze lines, but on a vitrified porcelain the glazes craze in the same cross-hatched region. mattanddavesclays.com/Science/Stull.pdfI believe thinner glaze makes crazing less visible, but doesn't make the glaze fit the clay? Your pieces are looking great so I hope you can tame that wild goose.
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Post by Arnie Benton on Dec 14, 2015 14:12:25 GMT -6
Lots of good ideas.
In my next firing I'm going to leave the Zinc and glaze thickness the same and reduce the top temp by 5 degrees and the hold at the top from 15 minutes to 5. Might change cone 10 from 90 degrees to about 30, I will guess! More crystals?
I know that this clay is being fired to cone 11 plus without problems -
I don't understand the Stull.pdf - As for crazing with less glaze application - my philosophy - if I can't see it it's not there! It probably doesn't make the glaze fit any better - Just maybe the thinner glaze IS a better fit - it may have more of the Si/Al that leaches from the clay than a thicker glaze -
Arnie
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Post by tileman2 on Dec 14, 2015 17:27:57 GMT -6
Sort of interesting seeing everyone chime in on the possible variables: each making very valid points. Sorta have to chuckle a bit too; the throwers are making adjustments from vertical to horizontal, and the flat guy (me) is making the adjustments from flat to vertical. Another possible solution to plates: www.highwaterclays.com/index.cfm?fuseaction=category.display&category_id=247Tom
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Post by evan cornish-keefe on Dec 15, 2015 8:06:51 GMT -6
Arnie, I am being a hypocrite, working at cone 6 none of my clay is vitrified. It's less than 1% absorption, I was taught that it should be 0, but crystalline glazes are sacrificing durability for aesthetics anyway, right? The Stull chart maps silica and alumina levels with a set amount of flux. Based on the UMF, the silica and alumina levels will be calculated in relation to the total amount of flux. Crystalline glazes, and crazed glazes typically fall in the lower left corner with little silica and alumina, where the glaze is very fluid. Where a glaze falls of the chart can give a good prediction of firing temperature, whether it will be matte, semi-matte or glossy, and how durable it will be (so etching wouldn't work as well as you increase silica and alumina). The fluxes used have a large effect on the surface quality, color response, and crazing. I recently made a "snowflake crackle" glaze, which needs to be applied very thickly without running, so I made a glaze with lots of sodium (to promote crazing) and fair bit of alumina (so it doesn't run). I'm just regurgitating information here, if you want to know a lot more about Stull, the UMF and formulating durable glazes, there's a great online class: art.alfred.edu/academics/glaze-formulation.cfm
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Post by tileman2 on Dec 15, 2015 13:37:15 GMT -6
Arnie:
Rereading this thread and some other posts made along the same lines. Two suggestions: 1. As soon as your plates hit the leather=hard state and you can remove them from the throwing batt safely: place them on wax paper so they can move while drying to green ware state. 2. Place SiO2 under the foot while bisque firing so they can move during inversion temps. I keep some very cheap 120 mesh around for this purpose. Your ear will get use to the right ping for a good piece, and you will hear a thunk on cracked pieces: but also a clank sound if hidden fissures are present as well. Flat pieces drag while drying to green, drag more when firing to bisque, and drag a little more in glaze fire.
Evan: I read the links you post; keep them coming.
Tom
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Post by Arnie Benton on Dec 16, 2015 12:35:49 GMT -6
Evan - that course on formulating glazes is very tempting. I've been doing this stuff for so many years now - without a glaze calculation program - mostly by using Koz's strategy of 'just try it' and learn something and proceed. And some help from my friends. Hmmmm. Tom - My strategy for bisque firings has been to stack as many pieces as I can get into the kiln - plates and bowls 5, 6, 7 high - and fire away. Adding up my answer here with my answer above and I come to the conclusion that I've been pretty casual about many aspects of pottery making. Things like the ping test I quickly adopt. In the last firing all the bisque pinged and nothing cracked. But I continue to come up with interesting results - If you look back at the previous page, piece labelled 2 - before and after striking - I took that same bottom glaze - and added Co .4 and Ti 3 - and got the following result - This has not been struck, but it looks much more like piece labelled 2 after it was struck. Certainly the difference from piece 2 unstruck is - well, striking. Arnie
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Post by sherri on Dec 16, 2015 18:46:06 GMT -6
I was having refired 10 - 14" bowls and platters split at the quartz inversion temp - the glaze ran inside the cracks so I know it was on the way up. I changed my ramp from 350 degrees per hour to 150 degrees per hour to 1,150 degrees and then continued on up using the 350 ramp and it solved the issue so far.
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Post by mohawkpiper on Dec 16, 2015 21:48:31 GMT -6
Tom - My strategy for bisque firings has been to stack as many pieces as I can get into the kiln - plates and bowls 5, 6, 7 high - and fire away. That would most definitely give me problems. I can stack two just fine. Three will give me a cracked platter from time to time. 4 or more is sure to crack the one on the bottom. The cracks come out of the bisque. They are hairline, very difficult to see without putting stress (pressure) on the plate. I'm stacking 10lb thrown platters (not sure how much they weigh going into bisque, but definitely less...) i'd guess that is probably a good place to look for your problems. G
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Post by tileman2 on Dec 16, 2015 22:08:30 GMT -6
Arnie:
Not sure I would strike fire the most recent plate: looks mighty fine just the way it is. The attention is always on the glaze and bisque firing technique: which it should be. Although for two years I have wondered if the clay formulation itself is part of the problem. We are forced to use commercially available porcelain, or stoneware with engobe. I spent the last week breaking down the known clay recipes into percentages of the various elements. Today, I worked on custom blending three different porcelain bodies based on percentages of the three primary fluxes: CA, NA, and K. I also wanted to keep the Al2O3 down to the lowest point possible in order to control the glaze flow manually with additions. Have long believed the clay body has also been a variable; which could be made more conducive to crystalline glaze. About a year ago I played with additions to clay bodies: decided to just start from scratch.
Tom +
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Post by Arnie Benton on Dec 17, 2015 10:13:37 GMT -6
Sherri - I can't remember ever seeing a piece that cracked on the way up - my attempts to stop cracking have included slow rise and fall through inversions without success.
Greg - My "bisque stacking technique" comes from college and art center courses where I watched the instructors pile as much greenware as possible into each load. I do try to make my pieces on the thin walled side so they don't weigh a lot, and I put the heavier ones on the bottom of the pile - but that could account for some of the breakage I get - I'm going to examine the bisque and use the ping test to sort out the cracked ones - more carefully in the future.
Tom - the chemical analysis of the clays is way beyond my level of knowledge. And the equipment / physical exertion required is tiring, just thinking about it - In my younger days I mixed my own clay by hand but now I'm happy (or stuck) with using the stuff that comes wrapped in plastic!
Arnie
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Post by Arnie Benton on Dec 18, 2015 15:02:33 GMT -6
OK - I thought about leaving the last pictured bowl alone - but decided that what I'm doing is investigating the effects of striking - sometimes pieces do look better unstruck, but how do I decide that until I strike it? So, here it is - I think it does look better struck - now, comparing it with the piece #2 struck on the first page, you can see the effect of adding more Ti and Co to the bottom glaze - in the pictures I prefer this one - looking at them side by side 'in real life' it's a much closer call. Happy holidays everyone! Arnie
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