Friday, 25 February 2011

Test tiles tell us it's all Tosh.

Well, the test tiles are out of the kiln.  I wanted to make sure that the firing was just like any other, so i changed nothing.  The only difference being that I left the kiln a bit longer before opening it.  To pack out the kiln, I filled it with some beakers and a bowl using the glaze with the china clay in it.

One would hope that, under these conditions, the fault we're trying to fix would occur on the packing pots.  But it hasn't.  In fact, no sign of any peeling on any of the pots.

Anyway, here are the test tiles.  A few nice ones, and a few duffers.  The main duffer was 70% LBS, 30% hyplas, which was opaque.  A good start.

I put some cones in the kiln.  I had modified the firing slightly, a few weeks ago, and it is clear that I am now only getting cone 03, with cone 02 half down.  Well, that's ok.

My favourite of the tests was the one I was most trying to avoid - the standard honey glaze, 75% LBS 25% body clay.  The second tile has the black slip on it.  It isn't black with the glaze on it - it's a dark brown - but the slip is black where the glaze isn't on it.  Which is good, because I was going to draw through the glaze, a la Cardew.

The next two - 5 (80% LBS 20% AT) and 6 (90% LBS 10% AT) are pretty good.  I prefer number 6, but not sure that 90% LBS is a good idea.

The next two are honey glazes of a sort, but quite light.  One is 80% LBS 10% red clay 10% nepheline, and the other is 60% LBS 10% borax frit 15% red clay 15% hyplas.  They're both quite nice but they're a bit streaky.

Here's the packing pots.  Not that nice.

Look at this beaker - this is the china clay recipe.  It's gone cloudy.  Quite surprised by that and not entirely sure why.  Well, it doesn't matter anyway, because it's getting thrown away, when I can work out where I'm going to throw it.

Well, not sure I've really solved anything, but I am going to mix some of (1) and either (5) or (6).

I've been looking at Dorothy Kemp's odd slipware book.  She doesn't mention shelling at all, and says that the glaze pings off if you open the kiln too early.  She also suggests a recipe for borax frit, which is just a mix of the frit and body clay.  I might try that, just to see what it's like.  I have some test tiles left.


  1. These tests all look very good Andrew. I agree that some are better than others and you now have some to choose from.

    I've not been keeping up v. well, but did you decide for sure that it is the glaze coming away, not the slip and the glaze? ie it's not a slip problem?? Just checking.

    The glaze I use is not 100% reliable. I found it randomly crawled from time to time. Other people use the same glaze and didn't have the problem. I decided to start brushing in on and it's done fine. No crawling. So maybe yours is an application issue. Or maybe you'll just have to live with some occasional flaking. (No different than the occasional disappointment from too much as or cracking in the wood kiln?)

    What is this borax frit that you have? Does it have a manufacturer's name or #?

    What's wrong with those beakers???!!! Stop putting your pots down. They are very nice. Tell someone how nice they are and then sell them to that person! :-)

    Okay gotta go decorate. Looking good. Keep it up.

  2. Hi Andy,
    Yes - it could be the slip possibly. What is the slip recipe? Re: the glazes, to stop shelling you need to add ingredients with higher thermal expansion ie. not borax but alkali frit. You are firing highish so the adding of neph. sy. or K feldspar could be good, even some whiting - as long as it dissolves into the glaze. You could also fire lower but am not sure you want to do that.
    Keep it up though - you'll get there in the end.

  3. Thanks Ron. No, not sure if it is a glaze or slip issue. I am going to try out a couple of glazes and see what happens. Perhaps test some alternative slip recipes too.

    There's no number on the frit, just a temperature range - something like 960 to 1200 or there abouts. I bought it to do some raku, but never did.

    As for the beakers, well, I suppose they're not nice in relation to some of the pots I've made and what I'm after, that's all. They're ok, but the glaze is milky and the slip decoration was done when the slip was too dry and so it is ridged. I prefer it when the slip is wetter, but I did these as a batch - slipped them all, then deco'd them all. All part of the learning curve, it seems. The bowl, on the other hand, was deco'd when the slip was too wet, but it's much nicer and the glaze is smooth and shiny. I like that. But I can only fire two bowls at a time as my kiln is too small.

    OK, hope your decorating is going well.

  4. Matt - I just copied recipes from a book - it wasn't a response to the issue, just seeing what other recipes do. Also, just wondering about whether lead is putting people off buying these pots. I am pretty upfront about what is in the glaze.

    I have thought about firing the glaze firing to a lower temp, but raising the bisque temperature (I think that's what industrial factories do - vitrify before glazing.) I might do a test firing like that and see what happens. The recipe with the neph sye is basically Paul Youngs recipe, except he uses K feldspar.

    Oh, the slip is just hyplas. It's what almost everybody who does red earthenware uses. I thought it wasn't the slip, but on the jars I fired the other day, it was clear the slip had pinged off. Thing is I do want a white slip - will i get that from AT, or am i talking about china clay again?

    Of course, my test tile firing didn't have ANY problems at all - bloody typical!

  5. hi andy, to get rid of a glaze safely, dry it ut in an old bowl, make sure it's dry then fire it up Makes it safe again. Same thing we should do with the contents of our clay traps -hmmmm!

  6. thats me not him by the way!!! hannah

  7. I'm not sure about e/ware slips but a bog standard white slip is often a mix of ball clay and china clay - 1/3 china clay 2/3 ball clay. Hyplas is good BUT it does have a high silica content, which could be causing problems. You might find the slips better to use with some china clay content. What do the other e/ware potters think?

  8. Andy,
    Try this slip it's easy enough.

    Ball Clay- 40
    Talc- 40
    Silica/Flint- 10
    Neph Sye- 10
    7% zircopax

    You may want to fire lower. Try for 04 down, 03 tipping.
    I bisque to 04, a bit hotter than most but it works for me.

  9. I was always taught 50% China clay for whiteness and 50% HVAR ball clay to ensure a good fit.
    Never been too sure what HVAR stands for though. maybe High viscosity something or other?
    I like the light honey glaze. It's not as stark as the transp and not as treacley as the honey.
    Could be your trademark glaze!
    purely our of idle curiosity, have you tried your glazes with a bit of oxide in the slip?

  10. Thanks H - I have a lot of old bisque pots suitable for that. Wondering whether to melt the glaze and open it up at top temperature and do a bit glass moulding - make a sand template or something. Hmmm, maybe not. Probably be too runny anyway. My clay traps are buckets - mostly festering and now lead contaminated.

    RE slips, thanks for all the advice. I'm not sure there really is an issue with the slip anyway, but plenty to chew on. I don't have any zircopax - i think that's a trade name anyway, isn't it (yes, zirconium silicate.)

    Interestingly, someone the other day suggested adding some of my body clay to the hyplas - and that's the advice Dorothy Kemp gives, about 1 part body clay to 10 parts ball clay, to give a cream slip. Perhaps it isn't such a silly idea after all. And if I use AT in my glaze, maybe that's the slip clay I should be using. Seems a shame to move from hyplas, the historical devon slipware clay, though.