: pricklypotter - Viewing Profile

yedrow, on 27 December 2012 - 10:07 PM, said:

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.plainsman...m/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'

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)

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.

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.

oldlady, on 16 November 2012 - 09:59 PM, said:

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.