k77

Hi all, A quick update. I sanded the pots and refired. The bumps were white and hard in texture. Somewhat difficult to remove. I was able to cover them with glaze, but the finished results (where my brush strokes are visible) are not to my liking. I have put a fresh load in the kiln today (that I glazed a few days ago). I noticed when putting them in that there were some hard dots on one of the plates. These wouldn't bother me in a dipping glaze (they would melt and blend in) but I'm wondering if in Stroke and Coat, with its lack of movement, these are the culprit? Maybe they are bits of dry glaze that I've picked up during a long painting session? Maybe they are small pieces of dry glaze from the bottle (I do try and get every last bit out!) So, my working theory for now is that these lumps may actually be from the glaze. This time I've rubbed them out with my finger before firing. Hopefully I'll see better results after this firing! K

Thanks again Callie. That's a really helpful suggestion in terms of finding clay bodies. Hi Min, Yes it has 25% grog but it is really fine (0.2mm). It is called 'impalpable' on the website. I switched to this clay after finding that my usual clay body (without grog) was tending to crack. I hoped that this would give a little more strength and prevent the cracking. Am I mistaken in doing this?

Yes Callie, this makes sense. It's interesting what you say about Stroke and Coat being less mobile - I hadn't thought about that. Compared with other commercial dipping glazes I have tended to find that it pinholes less (which is why I started using it - even though to be honest, it isn't very economical!) However, that was on a different clay body (no grog). You are absolutely right about the lack of choice for clay bodies here in Europe. I'm British, but learnt ceramics in California. As a result, I tend to rely on lots of resources in English. Here in France, I've had the task of trying to translate everything, but also have discovered some things just aren't available. I haven't found any clays that mature at a lower temperature. The choice appears to be between Stoneware (gres) and Earthenware (faience) only. Nothing in between. The plates I've already made, can they be saved? Sand and refire to lower temperature?

They seem more like bumps to me. As if there are grains of sand under the surface of the glaze in places. There are however also one or two pinholes, which is something I've been battling for a while. These white bumps seem different from the problem I usually have!

Hi Callie, Thanks so much for taking the time to help me! I will check the kiln shelves, but I would be surprised if this is the issue, it hasn't been an issue before...... I didn't trim the surface of the plates as such (I hand build), but...... I did use a metal rib to smooth out the clay. Now I think about it, this did expose the grog on some occasions (which I did smooth out with a rubber rib - but maybe not enough?) Perhaps I should only use a rubber rib for this clay?? I usually use clay without grog so this is something I perhaps haven't been paying enough attention to. I'm certain this clay is the one with grog. It is part of the W11 series. W11 is without any grog. Then W11 25 02 has 25% grog at a a size of 0.2. Seems I need to pay attention to how I'm making the plates so as not to expose the grog. But do you have any other thoughts on the firing temperature. I've been surprised to realise that the suppliers list such inconsistent results. Thanks again K

Hi Hulk, Thanks so much for taking the time to answer (and in so much detail!) Although the clay is not poking all the way through the glaze, my sense is that the bumps are coming from the clay - rather than from the glaze. In which case, off gasing seems likely. I will try and change the firing schedule somewhat and add a hold in the bisque as you suggested. The possibility of it being too hot makes sense to me, it crossed my mind when I saw the results. However..... Having read your reply, I am now somewhat confused about the correct technical data for this clay. In fact I bought the clay from Cigale and Fourmi that lists the water absorption as 1.3% @ 1300. https://www.cigaleetfourmi.fr/terres-a-gres/3996-gres-blanc-w11-chamotte-25-0-02-mm-sac-de-10-kg.html Confusingly, when clicking on the link to the technical data on that page, the information is different. (2.7% @1300). Another of my suppliers has the same product but lists the water absorption of the clay as 1.2% at 1260. https://www.ceram-decor.fr/en/ARGILES/8989-GRES-BLANC-CHAMOTTE-25--002---W2502-.html I'm certain this is the data I relied upon when choosing the firing temperature. Knowing that my kiln fires hot, I chose 1260 in order to have proper vitrification and achieve porosity of around 1%. The technical details on that page list the water absorption as: 1000°C : 13.1% 1100°C : 7,2% 1200°C : 2,4% 1300°C : 0,7% I can't find the technical details for this clay on the Witgert link you sent. It seems I need to resolve this, as possibly a lower firing temperature might be the answer (subject to achieving vitrification). I'd welcome any other guidance or suggestions. Many thanks! Katherine

This is my first time posting here. I would so appreciate some input. I have been firing using my own electric kiln for a few months now and have been having problems with pin holing. I've previously used witness cones to verify the temperature of the kiln (I found it does fire hot and have taken that into account), I have slowed down my bisque firing and separately added a hold and slow cooling. In short, I've tried to follow the advice I've read on this and other forums and despite using various new firing schedules, haven't fully resolved the issue (though it is a little better). Today I have a new issue. A bunch of plates that came out of the kiln are covered in raised white bumps. Is this pin holing? Or something else? For these plates I used Witgert white stoneware clay with 25% fine grog : https://auxcouleursdargiles.be/terres-blanches/1751-witgert-11-sf-blanche-2502-10-kg-4250536100804.html I glazed using Mayco Stroke and Coat Seabreeze (3 coats applied by brush) My bisque firing schedule is 60 ph to 200, 100 ph to 700 then 40 per hour to 980. My glaze firing schedule is 250 ph to 1160, then 100 per hour to 1260 with a drop to 1160 and a ten minute hold. The pots were quickly rinsed in water the day before firing (as they came out of the kiln) and were lightly wiped with a sponge immediately before applying the glaze. Can I save these by sanding and refiring at a lower temperature? Thanks so much in advance for any input you might be able to offer. K