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birdypotter

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  1. It's a little unclear - clays here usually state a really wide firing range and say 'they start to vitrify' at 1200 for example, but obviously we know that starting to vitrify and fully vitrifying will be miles apart temp wise. I think this clay will probably be at peak vitrification somewhere around the 9/10 mark yes, though I haven't tested up that far. At 1230 on my current firing schedule it takes on about 1.7% during the absorption test. Scarva says the firing range is 1200-1290 so I assume it will be fully vitrified closer to 1290 for sure.
  2. Update: Glazed 8 thin cylinders heavily in the white matte/clear combination and did the freezer to boiling water tests on all of them aaaand.....nothing. No cracks, no crazing, no dunting. The only thing I've changed is the firing schedule as @Babs suggested - 100˚c an hour to 600 so this might have slowed the heating through cristobalite inversion sufficiently. I also slightly increased the hold time at peak 1230 as @neilestrick suggested and the cones suggested firing perfectly to cone 7 (slightly over towards 8 in middle) which is just slightly hotter than it was before. So perhaps it was the firing schedule and temp that caused the problem rather than the glazes. I move forward cautiously optimistic....
  3. No, not at all! With an inaccurate COE for glazes that aren't fully melted, is the true COE value likely to be higher or lower or does that depend purely on the individual chemistry? Or is it that the COE value just cant be relied on regardless cause it's not fully melted? Just wondering whether the 5.9/6.5 figures of the matte vs the glossy are even further apart than that which would explain the problems, or whether they are more likely to be closer and therefore not as much of an issue.
  4. Quick update for anybody who followed this and might have the same problems: A mixed bag of results! I tested the cylinders with different glaze combinations as follows: Black in/out Black out/clear in White in/out White out/clear in Clear in/out Grey (almost identical to black) in/out Grey out/clear in I changed my firing schedule to allow for a slow fire up as suggested, adding in a 100˚c per hour to 600˚c. Cool down was kept the same. I froze all cylinders overnight and poured boiling water in them straight from the freezer the next day. Not one has dunted! Which I'm actually a little disappointed with as I was hoping for a conclusive result to my problem. However, the additional segment when warming up may have solved the problem, or I may still have the problem and by chance it hasn't revealed itself in this current batch. So I will be making more cylinders and glazing all of them in the potentially problematic white/clear glaze combination to be absolutely sure it won't happen again. If it happens in one of these then I'll know there will always be a potential problem in a batch. However I have adjusted the white glaze and substituted some of the tin for zircopax...it has brought the COE up to match the inside glaze more closely and hasn't massively changed the surface quality...it has just made the white slightly creamier and little more yellow. I might try to work out how to bring up to COE another way so that I can stick with tin oxide. In terms of absorption of my current clay, ES5 Earthstone Original for UK potters, I redid these tests with 14 bars as opposed to 4 this time and absorption came back on average at 1.7%. Scales were checked and calibrated so pretty confident in this figure being correct and happy to stick with this clay for the current temps. So...nothing conclusive and a little more testing to be done to be sure!
  5. Ah I see, thanks for clarification Pres. I'm in awe of how you fire, I feel frankly amateur with a controller doing it all for me! I am able to program some extra cooling cycles in to my firing program. I drop the temp fairly rapidly from peak down to 1038 and then from there down to 760 I cool very slowly to achieve the semi matte surfaces, but perhaps I will consider slower cooling from 760 down too by adding in some more segments. Lots to try!
  6. Hi Pres, thanks for the advice. May sound dumb here but unsure as to what you mean by firing down and up firing? Though I do have a fair amount of this clay, I wouldn't be averse to changing if it is indeed the issue - stubbornness won't solve my problem! It's difficult to find a clay here that is clearly rated for cones 4-7, for example. Almost all clays here are advertised as having a wide firing range and vitrification ranges aren't usually advertised or recommendations are that 'the clay matures at 1200 and above' which is obviously totally inaccurate when you're looking at specific applications for the clay such as durable dinnerware. If my absorption tests come out as the same as last time - which is on average 2%, do you think it would be necessary to change? I know the aim is for 0% porosity with porcelain but I have read this is much less likely with stoneware and 1-2 is to be expected. I tested Earthstone ES10, which is a smooth, lower fire version of my current clay, for absorption when I first tested ES5 (my current clay) and its absorption at 1230 was about 0.3% ...much more appropriate. The only reason I didn't choose this was because I slab press my plates in plaster slump moulds rather than throwing them and wanted the slight groggy tooth of ES5 to aid with warping and lifting of the plates in the centre. I dry my plates on plaster bats to evenly absorb the moisture, wax the rims immediately after making to avoid them drying out before the centre and weight with rice bags to try and prevent lifting when drying them under plastic over 1-2 weeks. I've not been with this clay very long but that seems to have worked with the few I've made this far...so it may work with the ES10 - as you said Pres, testing testing testing!
  7. Not a dumb question, I might not just have been clear enough. I do my absorption tests on unglazed bars of my clay that have been fired up to my glaze temp I followed the Digital fire instructions - is that the correct way to do it? I'm wondering whether Babs is right it's moved in to an existing crack as there was another smaller dunting crack around the other side that I put ink in to and when I broke that portion apart it didn't have the same issue. But yes, it could definitely be an absorbency issue. However I won't know for sure until I've finished redoing absorption tests just to double check my figures! I will be sure to update when I've done these and the tests Min suggested.
  8. On reflection, the ridges on the black glaze I think are caused by the slight groggy texture of the body - I think they're the throwing/trimming lines as the pot still rings when flicked, there's no ink settling in to the cracks permanently (I tried with white ink on the black outside) and its started since I changed to this clay but have only noticed it now. They're more dents where the glaze is settled rather than cracks. I think I can fix that just by taking more care to burnish the surface after trimming with a rubber kidney and not leave the obvious scars of trimming in. However I think your suggestion of raising the firing temp is a good idea regardless @neilestrick. When I did my absorption tests on this clay it came about between 1.4% and 2.7% at 1230 but on average about 1.2% at 1240 which was way better and tells me more about where this clay is likely to vitrify - though my glazes were too affected at 1240 so I stuck to 1230. I already have a 10 min soak but think I might increase it to 20 and see what difference that makes. Eventually I might have to go to 1240 and start thinking about changing the glazes if it turns out not to be COE mismatch. Seeing as I think the issue is only on one glaze now the COE mismatch is still a contender, so will still do all the tests mentioned above. And I don't think slowing the firing a little on the way up can hurt to try @Babs so I'll give it a go!
  9. Absolutely. So easy to feel defeated with ceramics but the feeling of getting it right is worth the frustration of getting it wrong. I'll soldier on!
  10. What good timing Min, this pinged up just as I was reading this thread! I think you're right, there is a little bit of a gap in clay availability made specifically for cone 6. Every clay body seems to have such a wide firing range, and the Potclays data sheet for a promising clay at earthenware was listed as having a firing range of 1100 to 1220 but a vitrification range of 1180 to 1250, which falls outside the firing range. The wide firing ranges given and lack of information readily available on other clays that Potclays don't sell, i.e. the lack of ACTUAL vitrification temp is a bit frustrating. Soooooo slowly losing the will to live but back to testing to check the ink issue. When I washed the ink off the dunted crack I didn't just wipe off, I ran it under a tap at full blast....wondering whether I forced the ink inward. Either way it still absorbed so I will redo the absorption tests....using ohaus cs series and have just double checked to make sure they are calibrated with weights I bought for this purpose- they are. So at least it's not the expensive scales
  11. The clays listed that vitrify at 1220 are listed as earthenware - I've been working with stoneware but perhaps need to change to earthenware. Would you think 2% is too high an absorption rate for functional ware? Am I looking more for 1% or lower porosity? I've read differing opinions so not sure!
  12. Could it be that the cylinders haven't reached the same level of vitrification as the test bars did? Perhaps because they are larger? I read yesterday on digital fire that sometimes bases in particular don't reach the same cone as the walls and can be underfed, therefore under vitrified. Could that have potentially happened here? Think I need to test some more cylinders but for absorption as well. Babs, would you suggest just slowing down only on the way up to 600 or including a further slow down on the cool down? I cool down from 1038 to 760˚c at 60 degrees an hour to achieve mattes, do I need to add an additional cooling ramp on to the end of this? Just as a matter of interest, I was looking at a table of clays from supplier Potclays here in the UK - they give the firing range but also within that the vitrification range which is handy. None of the stoneware clays vitrify in the cone 6 or even cone 7 range...I imagine it might be a similar story for the clay I'm using - so how are people making functional ware the popular cone 6 (considering the John Britt book and Mastering Cone 6 glazes both talk about firing to 1220ish)? Am I missing a trick here? Or is there an accepted level of absorption for functional work that doesn't reach full vitrification? Anyway thanks for all of your suggestions and input. I'll test the cylinders and see if the glaze mismatch is the problem. Will also test the Pike's Oatmeal with a combo of zircopax and tin oxide to raise the COE to close of that to the inside and see if that helps without changing the quality of the white too much. Will try to remember to update for any that find it helpful.
  13. Thanks Min, I'll try this. Assuming I did the absorption test correctly and got the correct figures (boiling for 5 hours, soak for 19, weighed before and after and calculated), how is the ink soaking in that much if the cylinder was fired to the same temp as the test bars? unless 2.1% is still far too much in terms of absorption?
  14. A frustratingly wide pool of potential issues and solutions. Firing higher would mean changing the glazes, firing to 1230 would mean changing the clay. Unsure as to whether the COEs of outer and inner glazes are too mismatched either and that has caused dunting. I'm at a loss!
  15. Hi Roberta Yes with my previous clay I had similar issues - it was absorbing too much water which definitely could have led to cracks. However though the firing range for this clay is pretty wide which does cause problems, I did some absorption tests and found that this absorbed about 2.1% on average at 1230˚c, which is fairly good in comparison to my previous clay. I'm hoping that this isn't an issue or I'll be looking at changing again!! (I've linked the thread that had those issues at the top of this thread if you want to have a look, might shed some light). Hopefully it fired to 1230 gives it enough vitrification, but who knows!
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