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Removing Scum From Fired Terracotta


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#1 Leslie

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Posted 10 May 2010 - 12:55 PM

Does anyone have a suggestion for removing - or at least mitigating - scum on fired terracotta sculpture? I usually only use an oxide wash and then refire, using little or no glaze. I have two pieces that I've just bisqued, and they seem to have much more scumming than I've seen before. I've searched the web, but can only find suggestions for preventing it happening. But its already happened and I'd like to salvage the pieces.

#2 Marjorie

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Posted 13 May 2010 - 11:12 AM

Does anyone have a suggestion for removing - or at least mitigating - scum on fired terracotta sculpture? I usually only use an oxide wash and then refire, using little or no glaze. I have two pieces that I've just bisqued, and they seem to have much more scumming than I've seen before. I've searched the web, but can only find suggestions for preventing it happening. But its already happened and I'd like to salvage the pieces.



There is nothing that will remove the scumming after the firing. I have smoked fire the pieces to cover-up the scumming and discovered that the smoke firing enhanced the over all look. Give it a try.

#3 Leslie

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Posted 12 June 2010 - 12:17 PM


Does anyone have a suggestion for removing - or at least mitigating - scum on fired terracotta sculpture? I usually only use an oxide wash and then refire, using little or no glaze. I have two pieces that I've just bisqued, and they seem to have much more scumming than I've seen before. I've searched the web, but can only find suggestions for preventing it happening. But its already happened and I'd like to salvage the pieces.



There is nothing that will remove the scumming after the firing. I have smoked fire the pieces to cover-up the scumming and discovered that the smoke firing enhanced the over all look. Give it a try.



Thanks, Marjorie. I did extensive research online. I was able to find some information about cleaning scum from (architectural) bricks. A mild acid was suggested. My pieces had a very heavy deposit of scum. I used vinegar - straight - and a scrubby, and a lot of elbow grease. It definitely cut down on the amount of scum visible on the surface. I coated the pieces with an oxide wash and refired. Also read about lightly oiling the finished work, but didn't need to do that. The finished pieces have been sitting almost a month now, and so far look good. I will be much more careful about the water I use in the building process. I don't think that the problem is with the clay, as I purchase commercially prepared clay (and hope I can trust it!). I will assume it's my water supply - which has changed recently - and I will use distilled water in the future. I will also keep your suggestion in mind. - Leslie

#4 sandy miller

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Posted 23 June 2010 - 05:35 PM

I just went through this with a terra cotta clay. Not sure which one you're using but I ended up firing to cone 2 and the scum was gone. The terra cotta lost the nice orange color I was hoping for but it's now a toasty brown and tough as nails. I made garden lanterns.

When I talked to the tech at the clay company I had ordered the terra cotta for schools, no where on the webpage does it say the clay is for schools. There is no barium in the clay body which prevents scumming, it's just a small percent but enough to make a big difference in the final work.
Good luck.

#5 Leslie

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Posted 23 June 2010 - 06:12 PM

I just went through this with a terra cotta clay. Not sure which one you're using but I ended up firing to cone 2 and the scum was gone. The terra cotta lost the nice orange color I was hoping for but it's now a toasty brown and tough as nails. I made garden lanterns.

When I talked to the tech at the clay company I had ordered the terra cotta for schools, no where on the webpage does it say the clay is for schools. There is no barium in the clay body which prevents scumming, it's just a small percent but enough to make a big difference in the final work.
Good luck.



#6 Leslie

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Posted 23 June 2010 - 06:18 PM

Thanks, Sandy. I'll check with my clay supplier about the barium. I, too, fired the next batch (already drying as the first group with scumming was discovered) to a higher temp. - cone 1-2. My experience is the same - deeper color and no problem with the scumming. I'd like to think I can trust my supplier when this problem is pretty well known and easily remedied. That's the reason I thought I'd suspect my own water supply first. However, I think you're right and I should check with the supplier. Thank you, and good luck to you too.

#7 Linda Rosen

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Posted 29 June 2010 - 06:36 AM

I have experienced scumming on cone 6 clay as well as on earthenware... I think that this is very likely due to suppliers moving away from adding barium to their mixes. Slow even drying helped ( the worst crud seemed to form on exposed areas of poorly covered pieces) . Check the work carefully before firing for any sign of surface scum and scrape off before firing.



#8 Leslie

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Posted 29 June 2010 - 08:05 PM

Thank you. I'll try this as well.




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