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eugene barnett

Glaze Fit Issue On Terracotta Slip

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For flower vases I’ve been experimenting with Standard Clays Terracotta Slip – C/06-04 with Duncan low-fire gloss black glaze (C/06) as a liner (Duncan IN1613 Black).

 

The problem: although there appears to be a good coating of liner glaze the pieces ping significantly when first out of the kiln and then water leaks slowly when filled. Any ideas?

 

http://standardceramic.com/CastingSlips.html#Terracotta

 

 

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The slip is not vitrified and the glazes, while delaying seepage, do not compensate for the lack of vitrification. Standard shows an absorbency rate of 5% for the terra cotta slip. That is still rather porous at cone 04. For water-tight, you need to get to 2% or less for absorbency -- preferably in the 1% range -- that is my comfort level for vases and other forms holding water for long periods.

 

Are you just using a liner glaze? or glazing both interior/exterior? You could try bisque firing at higher temperature -- maybe consult with Standard to see how far you could push the slip body. Some folks working in terra cotta apply terra sig to the bottoms of pots to help seal the clay body; again, that can delay seepage but it will not compensate for an unvitrified clay body.

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Your glaze is probably crazed, allowing water to seep through.

 

You might get a better feel for what's happening by enhancing any crazing with Indian ink (paint on, wipe off). 

[Certainly helped when developing a crackle glaze, fresh cracks can be surprisingly hard to see.]

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Thanks for the suggestions. It was also suggested that i try firing a few samples to C/1 (oxidation). My initial samples mentioned above were fired to C/06. The C/1 results are a deep red/brown I really like. The commercial glazes mostly bubbled with the exception of a Duncan Coal Black, although it may have micro-bubbling. There is a texture that suggests that Is the case. It's hard to tell because it's on the inside of the vases. I might just have to break a vase to have a close look.

For a better liner I'm also looking at glaze recipes that mature at C/1. If anyone has recipe suggestions, I'm all eyes.

Lastly, I am researching low-fire casting slip recipes with a Redart base. I think i already know the answer to my next question but here goes: do folks think I should jettison the commercial products and come up with a C/1 terracotta casting slip and base-glaze combo that play well together?

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What cone are you firing to for your bisque?

Cone 05: I had some pinholing in the first samples that were biqued to C/06 and went on to glaze-firing at C/04. I bisqued the next batch to C/05 in case the pinholes were from off-gassing. The second batch was better but the crazing issue became the bigger issue.

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