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Post by pribz17 on Feb 12, 2017 20:37:28 GMT -6
I am running into issues with pinholes. Has anyone had success refiring when this happens?
I fire bisque to 05 and will try going hotter next time.
Could finer silica cause pinholes?
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Post by Arnie Benton on Feb 13, 2017 11:41:18 GMT -6
Since the glaze completely melts in a refire it shouldn't be a problem.
My refires almost always produce an excess of small crystals - so I may cover them with a glaze with less Zinc - maybe 14 to 16% -
My usual caveat is that I do mostly horizontal surfaces with little glaze runoff - with vertical surfaces the amount of new glaze you apply depends on the amount of runoff from the first firing.
Applying new glaze to an already glazed piece requires heating the piece, so the new glaze will adhere and not just run off.
With a vertical piece I'd probably start by just putting a piece with pinholes in my next glaze firing and see what happens - that'll give you an idea about how much new glaze you many need to apply in the future, and whether the new glaze should have less Zinc.
Arnie
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Post by tileman2 on Feb 21, 2017 21:44:50 GMT -6
Pinholes are caused by off-gassing of feldspars- KNAO : either potassium or sodium. Potassium and sodium are in a gaseous state, and lithium and calcium are in a liquid state. Lithium is the primary flux in crystalline glaze; so the pinholes are not from the glaze recipe. Frit has already been melted once: so the sodium or potassium used to make frit has already disassociated as well. So this means your pinholes are coming from the potassium/sodium in your clay body. If you are using porcelain; sodium is the likely flux. If you are using stoneware, then potassium is the likely flux.
From 2050, and up to 2190F is when off gassing from the clay is at its peak. The thicker the clay wall is; the longer it takes for the kiln temperature to reach the interior of the body. The problem is compounded by the crystalline glaze firing ramp: which even towards peak is pushed as fast as the kiln can climb. So while the face of the clay is subjected to the heat your controller says it is: the interior of the clay body is not. In a cone 6 firing, adding a heating segment from 2050 up to 2230F @ 130F will resolve pin-holing issues. It allows the clay to completely off gas; thereby resolving your pin hole issues. However, additional heat work is also done by this slower heating segment, so you may need to lower your lithium additions slightly.
Tom
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Post by pribz17 on Mar 4, 2017 13:10:47 GMT -6
Thanks Arnie and Tom. It makes sense to slow down the last few hundred degrees because my firing schedule is going faster with newer elements. I will try this. I refired my pieces which eliminated all the pinholes.
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Post by Arnie Benton on Mar 4, 2017 14:28:45 GMT -6
I have a strategy for firing that has worked very well for me. -
Even with new elements I program heating rates that the kiln can easily handle - and cooling rates as well - so the firings are identical whether it's winter or summer or whether the elements are starting to go.
That, plus a cone 10 cone, plus recording the total time of the firing, gives me assurance that whatever results I get, it's not the firing schedule that's responsible.
Arnie
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