SweetheartSister Posted November 19, 2021 Report Share Posted November 19, 2021 Hi everyone, I'm making some porcelain Christmas decorations and was quite surprised/ disappointed to find that some of them have this horrible bubbling/peeling off situation going on. I've made 24 decorations in total, using lots of different underglaze colours (Amaco Velvet) and it is only the green underglaze which seems to be having this problem. A very small amount of pink has done it too, but not much. The decorations that haven't used these colours are 100% fine. I have just put them all in the freezer and in the morning I'm going to do the pour over with boiling water test to make sure that the ones which look okay, are in fact okay. I'm just wondering what might have caused this? I'm going to assume something wrong with the green underglaze. I have used said underglaze before with no problems...can they go off/expire? I am also using a different clay than usual - 99% of my work is made by slip casting porcelain, these are pushed out of a mould using slab ming porcelain. So maybe there was something this clay didn't like about that underglaze. Being my naive self, I just assumed that all porcelain would behave the same. It is worth noting that I also had some other pieces in the kiln made from my usual porcelain slip - all completely fine. I didn't use the green on those though so I can't compare in that sense. My firing schedule: Bisque fire - Cone 04 Underglaze fire - Cone 04 Overglaze fire - Cone 7 Any advice is very much appreciated. This is the first problem I've had with ceramics in about 2 years, so I should probably consider myself lucky! Link to comment Share on other sites More sharing options...
Min Posted November 19, 2021 Report Share Posted November 19, 2021 1 hour ago, SweetheartSister said: I am also using a different clay than usual - 99% of my work is made by slip casting porcelain, these are pushed out of a mould using slab ming porcelain. So maybe there was something this clay didn't like about that underglaze. Not all underglazes fit all claybodies, porcelain or otherwise. I think you demonstrated the pink and green don't fit the Ming porcelain. Underglaze is shivering off which means the coefficient of expansion is too low for this particular body. In effect the underglaze is too big for the claybody and is getting pushed off. Try tapping with something dense like a screwdriver handle other areas (especially on edges or corner) of both the pink and green areas, could very well be more shivering. Shivering can also take place weeks or months later so it would be best to scrap those pieces with those colours. Link to comment Share on other sites More sharing options...
SweetheartSister Posted November 19, 2021 Author Report Share Posted November 19, 2021 1 hour ago, Min said: Not all underglazes fit all claybodies, porcelain or otherwise. I think you demonstrated the pink and green don't fit the Ming porcelain. Underglaze is shivering off which means the coefficient of expansion is too low for this particular body. In effect the underglaze is too big for the claybody and is getting pushed off. Try tapping with something dense like a screwdriver handle other areas (especially on edges or corner) of both the pink and green areas, could very well be more shivering. Shivering can also take place weeks or months later so it would be best to scrap those pieces with those colours. Thank you so much for your response, that makes sense. I have put them all in the freezer tonight and tomorrow I will pour boiling water onto them to see if any more pieces ping off. I'll try the screwdriver trick too. For future reference, is there any way of knowing if a glaze and a clay will be mismatched, or is it simply a case of doing test pieces? The only thing throwing me off a bit is that I also made a much smaller test piece from the ming porcelain, using an ivory Amaco glaze. On this piece, the glaze once again ruptured on the head of the cat and has a couple of 'pimples' on the underside of the head. The weird thing about this is that I used the same ivory on many of my decorations with seemingly no issues whatsoever. I have attached a photo. The last thing that pops to mind is that upon looking at the website page for the clay, it suggest bisque firing to cone 06. I have always bisque fired my porcelain to 04 in the past, but I wonder if maybe the clay isn't adhering as well as it could be. I don't know if the difference in temp between 04 and 06 would make that much of a difference - I have a lot to learn. Link to comment Share on other sites More sharing options...
oldlady Posted November 19, 2021 Report Share Posted November 19, 2021 not answering your question, just saying congratulations on a beautiful job of painting and color selection on your very attractive decorations. i envy your skills. Link to comment Share on other sites More sharing options...
SweetheartSister Posted November 20, 2021 Author Report Share Posted November 20, 2021 31 minutes ago, oldlady said: not answering your question, just saying congratulations on a beautiful job of painting and color selection on your very attractive decorations. i envy your skills. Thank you, that is lovely of you to say. Makes me feel slightly less rubbish about my failure with the glaze on these pieces! It is such a shame when our hard work doesn't turn out as we had hoped. Link to comment Share on other sites More sharing options...
Babs Posted November 20, 2021 Report Share Posted November 20, 2021 I have had underglaze do this when the piece is too dry at application time. It looks fine for the bisque then bloats or ruptures when glaze fired Link to comment Share on other sites More sharing options...
SweetheartSister Posted November 20, 2021 Author Report Share Posted November 20, 2021 3 hours ago, Babs said: I have had underglaze do this when the piece is too dry at application time. It looks fine for the bisque then bloats or ruptures when glaze fired Hmm, that is interesting. All of them had the underglaze applied after a bisque fire, so the clay would have been an equal level of dryness across the board. This is why I'm thinking it must be an issue specific to the couple of glazes that ruptured. Link to comment Share on other sites More sharing options...
Babs Posted November 20, 2021 Report Share Posted November 20, 2021 Right After bisque. Dobyou wipe them with damp cloth prior to application?. Dust, or oil from hands? Difrnt thickness of application. Doe it happen if on a flat test tile I wonder. Link to comment Share on other sites More sharing options...
SweetheartSister Posted November 20, 2021 Author Report Share Posted November 20, 2021 47 minutes ago, Babs said: Right After bisque. Dobyou wipe them with damp cloth prior to application?. Dust, or oil from hands? Difrnt thickness of application. Doe it happen if on a flat test tile I wonder. I didn't wipe them - sometimes I wet sand my pieces but I didn't with these. Maybe that could have contributed to the issue. I left them all in the freezer overnight and poured boiling water over them all this morning straight away. There are no change - no crazing or anything like that. I knocked off the pieces that had risen off with the back of a screwdriver as Min suggested. Given that I did a thermal shock test and there was no change, no pinging or crazing, would it be safe to sell the imperfect ones as seconds after touch ups? I was thinking of embracing the mixed media look and adding textiles elements like little fabric patches and ribbons on the bare parts. But I would obviously not want to put anything in my shop that is going to be dangerous. The ones that I have whacked with the screwdriver, I have hit pretty hard all over and only the bubbled parts have come off. The rest seems stuck on very good. I was thinking to acrylic paint and epoxy glue over the chipped parts, then apply my fabric elements over the top. They will still be seconds but much better than throwing in the bin I think. I have also uploaded a picture of my green paint - it was well mixed before use but does appear to have little gritty bits in it. Could they have perhaps caused the problem? Link to comment Share on other sites More sharing options...
Babs Posted November 20, 2021 Report Share Posted November 20, 2021 I'd be sieving that. Consistency may be an issue. Thickness of underglaze coat. Could be inadvertently laying on thicker in areas which are problematic. Wear gloves so no oil on hands and damp sponge so no fine dust. As for selling after mending. Could be done, if you as an artist happy with the results. Link to comment Share on other sites More sharing options...
Pres Posted November 21, 2021 Report Share Posted November 21, 2021 Sorry, I didn't catch this earlier, but I believe it best under the Glazes grouping. best, Pres Link to comment Share on other sites More sharing options...
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