Jump to content

Clay Contamination Or Something Else? (Long)


Recommended Posts

Note: This post discusses an Australian cone 10 body few will know anything about - I hope it will still be possible to discuss generalisations and brainstorm causes! :)


Ok, my local pottery use a fine, white, 120mesh, high-fire cone 10 porcelaneous stoneware clay (Walker Ceramics PB103), this has been the primary throwing clay for at least the last decade. This year, almost every glaze firing has produced ware with some kind of surface debris of a tiny (1mm) to small (8mm) size. The debris appears on the unglazed pots, as well as scattered below every glaze, but it doesn't break the glaze surface. When you see the debris on the unglazed part of the pot, amidst the off-white clay body, the debris is bright white in colour, and looks for all the world as if you could flick it off with your nail. Sometimes the problem looks like a round pimple, other times it is completely irregular in shape.


The pieces are mostly utilitarian ware, bowls, cups, plates, some decorative and sculptural. Regardless of whether the surface has been untouched from wet/trimmed at leather hard/or sanded/sponged from bone dry, the debris appears. The biscuit firing shows no issues, but almost every glaze fired piece shows symptoms (but not *always*!)


After the first instance of this problem, the recycled clay anyone was using was dumped. Yet firings from the new bagged clay showed the same issue. (We were monitoring the clay being used carefully, as we were all working toward producing ware for a fundraiser.)


We are quite perplexed.


The group are mostly old school potters, I mention this because the internal discussion about our problem has no consensus. While there are a lot of folk with knowledge of the body, the kilns, and the process, no can agree whether the body is at fault, or something else. I am young, technically-minded, and have OCD - I want to understand and know why! While I know my clays, I know less about their kilns and firing.


Is is the body?

Is it dirty shelves?

Is something inside the kiln deteriorating?

Is it organics not burning out?

We wondered about over-firing, but we did a few firings where the cone 9 did not go flat, so over-firing seems unlikely.


The pottery is communal, different glazes are used, sometimes no glaze, and still the debris. We work exclusively with high-fire stoneware or porcelain. No low or mid-fire bodies are allowed inside the studio, or their electric kilns. No personal glazes, no experimentation, they risk nothing. I know little about the firing specifics, but apparently no firing programme changes have been made. I know they fire at a rise of 150°C per hour, top fire to 1280°C.


I know this post is all over the place and a bit rambling, and I apologise. I just want to know if anyone has seen or heard of anything like this before? :)


(I have enquired with the manufacturer, and am taking my dSLR to get detailed shots of the problem to send to them, when I have them I will post the images here.)


Many thanks. :)



Link to post
Share on other sites

I added some photos for reference, but now have a solution.


I spoke to the manufacturer, and the clay has undergone several minor reformulations, and is now 'higher in salts'. A few people have reported the issue since late last year, but a major reformulation is coming as the particular clay deposit is almost depleted.


The solution to overcoming the problem is to bisque for a minimum of eight hours, pushing the temperature to 1080-1110C rather than the 1000C we use at present. Additionally, soak at top temperature for 0.5-1 hour.


I'm glad I have a solution, but the manufacturer confirmed the body is now less white (amongst other things), which is a bit disappointing. Hopefully they will address that.




Thank you all for the brain power... :)


Link to post
Share on other sites


This topic is now archived and is closed to further replies.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.