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Glaze Pinhole Causes and Remedies


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Wollastonite being better than whiting because it has less LOI is just a myth... Even at cone 6, cones are designed to measure the last 100 C of the firing for a reason, below that point, they just don't melt. If you can prove (scientifically of course) that a single material melting is going to trap gasses from whiting please publish it and let me know. Common sense tells me that it won't, even if that material is 30% of the recipe, as long as there's room for the gasses to escape (and there is, you can check the article Melting behavior in Traditional ceramics) this shouldn't have any effect on the result. I'm not saying that I'm right and you are wrong.. but I think that some statements should be taken with a grain of salt, because they are not proven, and when you have a problem like the op has, this wouldn't be the first thing I try. 

So going back to the OP's problem. I see a very strange color in your test tile, did you add a colorant to the recipe that you didn't tell us about? Are you sure your kiln reached temperature (did you use cones)? If you only have problems with this material, have you tried using it in other glaze recipes? Maybe you just got a mislabelled batch (this has happened to me before). Those bubbles really look nasty. 

Look at this picture from Derek Au (creator of glazy):

On Standard 130 Porcelain, no colorants

same glaze... using whiting, no bubbles or pinholes.

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4 hours ago, thiamant said:

you can check the article Melting behavior in Traditional ceramics

Can you be a little more specific?

PS Best I could find was:
T. Lam, G .L. Wynick, and W. M. Carty, “Melting Behavior in Traditional Ceramics”. J. Am. Ceram. Soc. 90 [3] (back cover). Publications. 5 pages
... although the significance of "(back cover)" escapes me

Expecting to be able to access the abstract online I looked at  vol 90 issue 3 and failed to find any trace
https://ceramics.onlinelibrary.wiley.com/toc/15512916/2007/90/3

An author search for the most distinctive author name returned
0 results for "Wynick" in Author published in "Journal of the American Ceramic Society"

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Wollastonite

  CaO 48%

  SiO2 52%

 LOI (1000C): 0.5% (Vansil: 1.6)

Whiting

  CaO 55%

  Traces MgO, SiO2, etc.

  Which one? Digitalfire lists several; LOI ranges from unspecified to ~43%; GlazeMaster lists Whiting with 44% LOI

The tin/chrome red I'm using* includes twenty parts whiting; it goes on well, melts well, doesn't run, and I'm not seeing any pinholes over any of the clays I've tried. It's been a great glaze (so far) - one of three that I'm stirring, sieving, and applying each glaze fire (Lakeside Pottery's Clear Blue and Bill Van Gilder's Rutile Green be t'others). This comparison may help me, however, as I've just over five pounds of whiting left, and most of a fifty pound sack of Wollastonite, hence juggling the recipe for Wollastonite may be a good exercise!

*Credit local JC Ceramic lab for the recipe - which is same as "Chrome Red" per John Britt.

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Trying to broaden this question out to include more than LOI as a possible culprit for pinholes. It's been my experience that pinholes can be caused by a number of things. Since one glaze high in calcium carbonate won't have a pinhole issue and another will there are other factors in play. 

A few things that I've found contribute to pinholes from bisque to glaze fire. Groggy claybodies more prone than smooth or porcelain claybodies. Clean bisque firing necessary to fire out organics etc from clay to prevent claybody gasses being expelled through the glaze. Glaze application. Dampen the bisque ware if necessary to help prevent air bubbles escaping from the bisque, especially on trimmed groggy bodies. Raw glaze thickness, avoiding too thin a glaze application or overly thick and also glaze laydown. Under or over firing the glaze. Fire the clay to its maturity and adjust the glaze if necessary. High viscosity glaze recipes have a more difficult time healing over the pinholes during the firing, fluid glazes not really an issue. Large amounts of LOI materials such as dolomite, calcium carbonate, zinc, rutile in a viscous / low fluidity glaze. Drop and soak/hold firing helpful in clearing pinholes.

 

 

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