Crazing Query

In article , potty writes

Yes, that is correct.

You need to modify the recipe for the Crank body or an entirely different recipe. One which has a lower coefficient of expansion. Do you know the calculated COE of the glaze that you are using? This is where a glaze program comes in very useful. First step might be to see what adding more silica does to the COE. Without knowing the recipe I can't help much more.

David

Linda's recipe is: whiting 27.3 feldspar pot 19.7 epk 20.1 flint 33 I'm using a standard china clay for epk as we don't get that in the uk. The expansion coeff. is 68.5 orthereabouts. I altered the recipe which now reads like this: woolastonite 10 feldspar pot 27 china clay 26 flint 31 calcium borate frit 6 I thought woolastonite would be a better way of introducing the calcium, KNa2O is more than double, the silica content is about double keeping the Si:Al ratio is slightly lower (7.44) and I've replaced some of the silica with boron, the exp.coeff. is now lower (59.2). I haven't tried it yet but I think it will work. What do you think?

Linda's recipe is: whiting 27.3 feldspar pot 19.7 epk 20.1 flint 33

I'm using a standard china clay for epk as we don't get that in the uk. The expansion coeff. is 68.5 orthereabouts. I altered the recipe which now reads like this:

woolastonite 10 feldspar pot 27 china clay 26 flint 31 calcium borate frit 6

I thought woolastonite would be a better way of introducing the calcium, KNa2O is more than double, the silica content is about double keeping the Si:Al ratio is slightly lower (7.44) and I've replaced some of the silica with boron, the exp.coeff. is now lower (59.2). I haven't tried it yet but I think it will work. What do you think?

pete

This is the second time I'm posting this as it seems to have disappeared from my system so that might be why you might be getting this one twice!!

Pete,

Your changed recipe certainly reduces the COE significantly and also keeps the Si:Al ratio similar which is good. I usually try to make as few changes at a time but I think it is worth you doing a test on a small amount.

From the COE figures that you quote, would I be correct in thinking that you are using Insight to do your calculations?

David

In article , potty writes

David

If I'm altering a recipe I usually like to go the whole hog applying the limited knowledge that I have with recommendations from experienced potters whose styles I like. I know that small alterations give you a better picture of the effects of individual ingredients but I'm not a chemistry major. If the software I'm using, which is Glazemaster by John Hesselberth, gives a chemical analysis of the original glaze, I can use that to go where I want to go by applying a few simple rules like increasing the ratio of silica over china clay keeping Si:Al the same to raise the melting point, or replacing some silica with boron to reduce crazing, swapping fluxes to effect meltingpoints etc etc.

Glazemaster has certainly done the trick as far as analysis and database functions go though it's a bit slow. It can do a whole lot more but I haven't had the chance to use it yet to its full capacity. Anyway it's the best one I could fing that works across both mac and pc platforms and, using Mac OSX, it's difficult enough finding compatible programmes for 'limited interest' subjects.

Pete

Srves you right for buying an apple.

Jw

not much choice there John I've been a graphic designer since the early nineties and ever since then right up to the present time you will not find any major designers of any sort using Windows (except maybe for word-processing and accounts). You might remember Betamax versus VHS - same argument, the better system cost more and wasn't as well promoted, but thankfully Apple Macs won't suffer the same fate.

Pete

Sorry about that. I must admit I was only stirring things up. Back to the clay.

Interesting little sideline though. One of my fellow students taking the glaze course from Otago Polytechnic had a difficult time becase her computer was a Mac and the software (Matrix) was PC. She eventually solved the problem by installing a program called Virtual Machine, but has had all sorts of problems with her peripherals.

John

that's cool John unfortunately, those pc emulation programmes are crap and we Mac users sometimes get very frustrated with the lack of attention from software developers. It's usually small operations that can't afford the time or cost of writing programmes for more than one platform but we just have to be thankful that they write them at all, and it's hardly surprising that the more popular pc alternative is first choice.

pete