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Gloss Glaze Comes Out Matte


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#1 MarkS

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Posted 26 April 2011 - 08:48 AM

Just took a load out of the kiln yesterday and noticed that several pieces didn't have a nice glossy finish.
I was using the Amaco PC temmuko glaze over ^06 bisque fired Highwater Clays brownstone then fired to ^6 (by kiln sitter but that's another discussion).
Didn't seem to matter where it was located in the kiln - bottom, top, center, outer.
Is it not getting hot enough or what's the problem.
Should I refire and if so should I recoat first?
Thanks!

#2 Marcia Selsor

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Posted 26 April 2011 - 10:50 AM

Could be the combination, could be on too thin, or it could be you need to soak the glaze at the maturing temperature. You could try using a ^7 is you have a cone setter. If it looks too thin, try adding more glaze. If you think you got it on thick enough, just refire it hotter or soak at ^6 for 30 minutes or so.
Let us know what happens.
Marcia




#3 Chris Campbell

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Posted 26 April 2011 - 10:59 AM

Judging by your kiln heat question i would say it was under fired for sure ... But I also agree with Marcia that there could be those other factors at work too.

Chris Campbell
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#4 MarkS

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Posted 26 April 2011 - 11:08 AM

Everything had 3 coats brushed on.

#5 Marcia Selsor

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Posted 26 April 2011 - 12:20 PM

Three coats is hard to interpret. But in your judgement, if you think you have enough glaze on the surface, the refire hotter. I would also try this glaze on a different claybody to see if there is a possible chemical reaction between the glaze and your clay with the undesired results. Clays can differ in their chemical content such as having high amount of alumina or just something that doesn't chemically work with a particular glaze recipe. Try this glaze with a ^6 test tile of porcelain or a different stoneware to see how it reacts.
Marcia

#6 azjoe

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Posted 26 April 2011 - 01:37 PM

Based on your kiln sitter question thread, I agree w/Chris it is likely your kiln didn't fire hot enough... are you sure all your elements are functioning correctly? Were these the only pieces in the kiln or did pieces with other glazes come out ok?

Regardless, once the correct temp is reached, the question of thickness Marcia raised will enter into this. FYI, if you look on Amaco's site on the Potter's Choice webpage it states in the "Here is how George describes how to control them through their application" section that you will usually get better results with 4-coats instead of three.

FWIW, anytime I'm trying out a new glaze I always fire a gaggle of test tiles made up of the various clay bodies I use. I prep each with a range glaze thicknesses to see how the glaze reacts... ie, the color/effects, breaking, sagging, running, etc. I can't think of a single incident where I thought a glaze looked good on every clay and almost all glazes "get ugly" at one extreme or the other of thickness. Granted, testing slows down the process... but I've learned on average the minor euphoria of getting a test tile I like is worth far more than the disappointment from a piece of pottery I liked being ruined by a new glaze that didn't fire on my clay/in my kiln with the same result the vendor's literature showed. YMMV


#7 Chris Campbell

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Posted 26 April 2011 - 02:46 PM

Except for a few potters, almost all of us hate to postpone the finished product by testing.
It's boring, repetitive and time consuming.
BUT ... I'm always surprised by the results and happy I did not leap into blind firing.
Still not fun ... But worth it.

Chris Campbell
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#8 MarkS

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Posted 26 April 2011 - 02:58 PM

Everything else came out pretty well. The pieces with this glaze don't really look bad - they actually have some character - just not what I expected.
But I am really interested in knowing what's going on.
I wasn't sure if it was a characteristic of this glaze.
I really haven't been able to get the "Ancient Jasper" to do what I wanted either.

#9 MadMudder

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Posted 06 May 2011 - 12:52 PM

Ahhh testing...
That is what I am SUPPOSED to be doing now but of course instead I am reading the forums.
I switched from Cone 10 gas firing that basically NEVER did what I wanted it to do to cone six electric. I have about 25 slips and glazes to test. Going to finish up the tiles and fire them tonight.
It will be the first time with a computer and I plan to soak it for at least a half hour after it reached temperature.
Should I soak it longer?
I want matt surfaces but also need the pieces to be well vitrified to be well vitrified.
This is a pain but will get me going. I am in my first show in North Carolina on May 14th. Doh!
It simply cannot be worse than that darned gas kiln I used to have. There was about a two shelf sweet spot in a forty cubic foot kiln. It made me insane...
B
MadMudder

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