Jump to content


  • Content count

  • Joined

  • Last visited

Community Reputation

1 Neutral

About pricklypotter

  • Rank
  1. The true meaning of COE

    the answer is yes. it's a coefficient of linear expansion (ie how much length, delta L, as a part of the original length L is added with added temperature, delta T). see for example http://www.plainsmanclays.com/data/H550.HTM or similar graphs is glaze chemistry books (i think there are some in MC6G) a little twist in this: you'll notice the number you're getting is for the area where the curve is a straight line, and that the curve is not straight at all points (ie at certain temperatures your clay/glaze will lengthen MORE with temperature change than at other temperatures. this happens for example around the quartz inversion and around cristobalite inversion, which are temperatures at which the clay/glaze undergo a sudden and bigger expansion). as for the number of zeros, it doesn't matter as long as you omit the same number of zeros from all such figures. Like writing 'Neph Sy 35' in a glaze recipe and meaning '35%' or '0.35'
  2. White glaze turning yellow

    Seems to me John is right about the thickness. Examining the picture closely, it seems that even in the bright yellow area the glaze is still white where it pooled (eg inside the seashell texture). And it does seem to break yellower on edges even where the rest is white (I also thought the glaze may be pulling iron from the body, but checking out the clay, it seems to be quite white)
  3. ceramic?porcelain?stoneware?

    I don't think ^6 porcelains are strictly soft paste anymore. They probably just use a lot of Neph Sy as flux instead of a little feldspar. I thought that strictly speaking soft paste is fritted. ??? if you define it by firing range then ^6 porcelains are soft paste but that would be a tautology. i think a good definition of porcelain is that it's white, translucent where thin unless glazed with an opaque glaze, rings when tapped, and is completely or almost completely vitrified (less than 1% or .5% or...). In terms of composition some people like to say that it contains feldspars, primary clays (kaolins) and silica, and no ball clay, but a lot of commercial porcelains do contain some ball clay. some of them are not translucent where thin. it's a hornets' nest stoneware usually contains some iron, and so is grey, buff, red, brown... but white/porcelaineous stoneware contains little iron. i think by stoneware people generally do mean a clay which is at least mid fired (cone 5 and higher?), but can be up to cone 10, that is not completely vitreous when mature (1% - 3%?) and that contains larger amounts of secondary clays. the border between white stoneware and porcelain can be a bit contentious.
  4. Final update, mainly for posterity (if anyone asks a similar question in the future) I have fired SPS grolleg and it's gorgeous. very white with a tinge of blue and very translucent. excellent glaze response. in this firing i also found a mug that was very grey on the bottom and wondered what it was until it dawned on me that I had traded someone some SPS grolleg for some SPS tosch to try out. that was my tosch mug. so to sum up, in descending order of whiteness and translucency: 1. SPS grolleg is hard to work with but beautiful if you can tame the beast. 2. SPS kutani is easy to work with and is a bit translucent where thin. it is a bit off white (greyish) but still has a very satisfying glaze response 3. SPS kenzan - everyone's go-to functional porcelain, can't say i have ever seen any light through it and it is decidedly off white. I can't say it's more workable than kutani. 4. SPS tosch - grey. looks almost like seamix. made the celadon look grey and dull. same workability as 2 and 3. and of course plainsman p700 which is a tad more blue than SPS grolleg and more tranlucent (and easier to get very thin, my recent 1lb mugs are 600ml, compared to 400ml with SPS grolleg) and aardvark tom coleman which i haven't worked with but is extremely white and a bit translucent too where thin.
  5. Food Safety

    The green/blue glaze: using copper carb lowers the effective amount of copper in your glaze. if you use copper carb and want to keep the same 'recipe', you should use more since copper carb "=" copper ox + CO2 that burns off in the kiln (LOI). i would go two ways: 1 does the glaze REALLY need so much copper and cobalt? the fact that you once used carb instead of ox and got the same colour response suggests that it doesn't. I would go in the direction of lowering the oxides and seeing if i still get the same colour response. cobalt too is a powerful colorant, 2.5% is a lot and you can probably do with less after lowering the amount of oxides, try to make the glaze base itself more durable: 2 also, add clay (= alumina) and silica to see if i still get the same surface and colour response each of those steps should ideally be done on a grid of 25-35 paper cup test batches, all test tiles fired in the same firing. in the end you will be able to choose a glaze that has the more or less same colour and surface but is safer to use and more durable too. no advice about the red. there is a certain kind of red that i wouldn't use no matter how many times they put 'non toxic' on the label.
  6. Some companies will have the numbers for you (if you mix your own clay, test, as said above): scroll down to 'fired absorption', defined as John said, and see. ^6 is NOT inherently less vitrified than ^10, as the numbers show: http://www2.plainsmanclays.com/data/M390.HTM vs http://www2.plainsmanclays.com/data/H440.HTM http://www2.plainsmanclays.com/data/M325.HTM vs http://www2.plainsmanclays.com/data/H550.HTM http://www2.plainsmanclays.com/data/P300.HTM vs http://www2.plainsmanclays.com/data/P600.HTM the numbers are more or less alike, and UNLIKE, for example, http://www2.plainsmanclays.com/data/L212.HTM which is not even close to vitrified even at ^2 (but is usually fired to ^02-04!). Red earthenware bodies seem to fare better and are as vitrified, at ^2, as ^6 stonewares are at ^6 but I don't know if this applies across the board. Ovenware has more to do with thermal expansion properties, again, supplied above, in some cases. The alpha-beta quartz inversion (573C = 1063F) is a concern with every clay body but home ovens don't reach temperatures that are even close. The cristobalite inversion is of more concern (220C = 428F, a hot oven setting reached routinely in home ovens) but not all bodies have that. It would show as a bump around 220C on the thermal expansion curve. Again if unsure, test. David
  7. I don't tend to make very tall stuff, just normal tall, bigger (2-4 litre) bowls, pitchers, 1 litre teapots, biggish mugs (~ 12cm high when fired) and the like. If you want to go taller I have no words of wisdom at all. How tall do you wan to go? I've definitely gotten Frost to be about 20-25cm, but then I usually open it up into a bowl. I don't know that I have any trick except do as few pulls as you can, not so much water, don't mess with it too much when you shape it, be obsessive about removing slip from the walls and bottom, throw with thin slurry and not water. the usual porcelain words of wisdom. maybe the wheel should be a tad slower than for stoneware or stuff like Bmix or P300. But not as slow as for dove, where the slightest bit of centrifugal force will cause it to flop. I find that Frost likes to hold its shape quite well, indeed that it is quite rubbery and tends to bounce back. I don't necessarily like trimmed feet on my work but with Frost I now trim everything since otherwise I find the bottoms crack too much from the outside. oh yes, if all else fails, liberal use of the heat gun. I now use it just to take pots off the wheel and not so much when I'm actually throwing but sometimes it just can't be helped, especially if you're throwing tall and taking many pulls to do it and the clay is getting mushier by the nanosecond. I hope you find some of it a little useful and not completely trivial. d.
  8. Executive summary so far Plainsman P700 is amazing. it is very plastic and very easy to get thin and big. i throw 0.5-0.75 lb mugs with it because the 1lb mugs come out huge. easy joinery for porcelain, no problems whatsoever. In addition it is a slightly bluish white at cone 10R and very translucent. Amazing pure glaze response. I am in love. Expensive stuff though. SPS Kutani is plastic, doesn't get as big as P700, but can take a lot of abuse. attachments crack more. It fires to a very slightly greyish white and is translucent where quite thin but not as much as P700. If you can get it in Seattle, it's less than half the price of P700, Greenbarn don't carry it. SPS Tosch feels like Kutani to work with, haven't seen the fired results. SPS Grolleg is exactly like SPS Dove to work with, ie horrible and limp, but more specifically with all the quirks of Dove. Try it out, you'll hate it as much as I do... I haven't seen the fired results yet, I hope they're worth it. I have three more bags and can't wait to finish them. It took me a day to get used to those quirks again, a less painful learning curve this time to be sure. But I don't enjoy it.
  9. Strontium matte questions

    Hi, I've tried this recipe for a cone 10 reduction turquoise matte (I found it online): Neph Sy 50 Strontium Carb 38 OM4 6 Silica 6 add: Copper Carb 3 I know it's low on silica. I quite like the colour, turquoise where thin and almost black and matte where thick, and the nice, dryish frosty surface. In my little line blend of adding the copper 1% at a time, the 1% tile reduced to a matte pinkish tone, the 2% tile partly reduced, and the best turquoise was at 3%. I was thinking I could add 0.5% to 1% more copper to see if it improves more. My problem is that it crazes on my clay bodies (plainsman P700 and SPS Kutani). The problem is visual since I am not going to use this in contact with food. I was thinking of fixing this in three different ways: 0. adding silica until it's within the acceptable limit, and risk making it shinier or somehow changing the nice colour response 1. adding (5%?) zircopax (this also adds silica) 2. substituting spodumene for some of the neph sy, to replace some (definitely not all!) of the KNa with Li. My direct question is how to do (2), would a mole of Li for each missing mole of KNa work? And the more general question is which of these approaches would seem more promising. Thanks! PS for (2) I thought of matching the flux mole for mole, then balancing out the remaining alumina with clay and finally balancing out the remaining silica.
  10. Once again: how do I reclaim clay?

    also: save your throwing slurry and add it to the recycling bucket, you'll be putting all the fine particles back into the bucket and the clay will be more plastic that way.
  11. Tall, narrow forms

    Another technical idea that works better, for maintaining fluidity of form, than 'sectional' pots (i heard this is how ming vases were made): throw the bottom, leave the rim thicker than you want, get to leather hard. now centre more clay and open to the wheelhead, making a fat coil that would sit on the rim of the leather hard pot. score and slip the rim of the leatherhard pot, recentre the pot on the wheelhead (a plastic batt would work) and attach the coil. throwing the very bottom of the coil down a bit helps strengthen the joint. now keep throwing the coil up, with slurry instead of just water so it doesn't wet too much of the leather hard pot underneath it. you can reach a very organic, seamless shape this way. if the joint bulges a bit you can scrape it away with a serrated rib and smooth it after. cover, with a fabric under the plastic to even out the moisture, and, in principle, when the top is leather hard, repeat ad infinitum. the only caution is not to do it just over a very abrupt change in the curvature of the pot. i don't make very tall stuff but i raku fired thin porcelain goblets this made this way and they didn't explode.
  12. I don't do sculpting but Dove - I look at it wrong and it flops. Maybe for sculpting it's different. It's definitely almost completely white. but I would just bite the bullet if all i wanted is to go in, get clay/raw materials, go out. This time I wanted information, and that was my problem. But in mid november I'll have the answer to my own question and post it here.
  13. maybe i'll try to go to tacoma next time we're down there. I just tried throwing a 2lb bowl with the Kutani and was impressed by how plastic it was and how it didn't flop even though i made it very thin and open. I'm looking forward to seeing the fired results - also made some little glaze testers that i can probably fire earlier just to see what it looks like and what it does with glazes. it looks darker than their grolleg body when it's wet but that doesn't necessarily mean anything.
  14. Thanks Peter, good advice. I've been thinking about shipping to Pt Roberts. I don't usually think in large quantities of clay but I guess could, and maybe should. D.
  15. Thanks for the porcelain comparison, and the stoneware too. It sounds like you have way more experience with SPS than I do, more of the same though Maybe I'll phone Clay Art and see if they can ship to Seattle, I don't think there's time to go there in person now.