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Glaze Pinhole Causes and Remedies


thiamant
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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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  • 2 months later...
On 8/30/2021 at 7:51 PM, thiamant said:

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.

I am learning gasses can chemically dissolve into the melt and it varies on the chemical composition. I think some gas from whiting can be trapped into the melt and then produce bubbles as gasses generally lose solubility as silica arrives and the temperature increases. (It actually increases in solubility with temperature, I got that wrong.)

548280286_Screenshotfrom2021-11-0118-53-50.png.fa736f24a58f4e68d59857c94eaa541c.png

 

Edited by High Bridge Pottery
Correction
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1 hour ago, High Bridge Pottery said:

I am learning gasses can chemically dissolve into the melt and it varies on the chemical composition. I think some gas from whiting can be trapped into the melt and then produce bubbles as gasses generally lose solubility as silica arrives and the temperature increases.

That does say “can” and “some”, do you think this is a significant contributor and or predictor for pinholes?  

Edited by Bill Kielb
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Looking forward to reading  the full Anna Wilhelmina Maria Wondergem-de Best ref for myself - not today, though, am into Andy Weir's Hail Mary, among several other things... 

Title: 

Redox behaviour and fining of molten glass

...yep, that's the actual title - available for download, one copy for research/study, etc.

Edited by Hulk
title
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18 hours ago, Bill Kielb said:

That does say “can” and “some”, do you think this is a significant contributor and or predictor for pinholes?  

I think it has to be pretty significant but it's only a working theory. A lot of the time grog can be the cause of pinholes. I think this is because the grog at the body/glaze interface is a good place for bubble nucleation and a source of silica to decrease the solubility of gasses in the melt.

11 hours ago, Hulk said:

Looking forward to reading  the full Anna Wilhelmina Maria Wondergem-de Best ref for myself - not today, though, am into Andy Weir's Hail Mary, among several other things... 

Title: 

Redox behaviour and fining of molten glass

...yep, that's the actual title - available for download, one copy for research/study, etc.

Sorry I should have posted the citation/link, I have only ready some and understood next to nothing :lol:

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In continuous glass production (as described in the thesis) the ratio of surface to volume (in the tank of molten glass) would be considerably less than the typical ceramic coating case, where the glaze layer is, what, ten to thirty thousandths or so - lots more area for bubbles to leave the melt, much less distance to travel.

The parallel that comes to mind, where a typical ten gallon aquarium has twelve square inches of surface for each gallon, hence the dissolved gas levels (O2, CO2, in particular) are driven by contact with the atmosphere, not the plant life within; compare to the local lake/pond where twelve square inches may correspond to hundreds, even thousands of gallons, or more.

 

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12 minutes ago, Hulk said:

lots more area for bubbles to leave the melt, much less distance to travel.

Seems logical. My knowledge stops at simple application of Henry’s law (not relevant here) but I think solubility very unlikely a predictor with respect to glaze firings. I will say I have seen videos of maturing glaze and they can be a bubbling mess at some point. I still think fired surface tension seems likely to have a significant role in whether they will heal or not.

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I do agree that the solubility is not a good predictor for pinholes and if they get stuck in the glaze or can escape and heal over is down to surface tension and viscosity.

What I am trying to say is the solubility of gasses in a glaze is a significant source for bubbles and some of that starts to dissolve long before 800c~ when the carbonates are decomposing as the OP is thinking.  If you look at the frit sintering and softening temps below they are all under 800c.

 

It's not an ideal to compare glass production with glazes as you say, the volume to surface area is completely different. A paper that seems to hit that problem is  Glaze Surface Tension Effects On Bubble Evolution by Liping Xiong where using Antimony III Oxide actually results in more bubbles in the glaze even though in glass production it's used to remove bubbles through producing more of them.

They do find good results with Molybdenum VI Oxide as that reduces the surface tension of the melt and changing the RO:R2O ratio.

In the RO:R2O experiments they produce some experimental frits and then fire the B1 frit onto three different substrates. Even though the frit is held at over 1350c for two hours, the only other material in the glaze is 8% EPK and the substrates have been prefired they all have so many bubbles either from trapped gas or dissolved gas in the frit.

1186395457_Screenshotfrom2021-11-0609-41-48.png.60bc4afc62ca391e88af98bcfa5887d0.png

 

1052279904_Screenshotfrom2021-11-0610-41-44.png.dd2841f05b1b64f13ca08c337c3da08b.png

839613146_Screenshotfrom2021-11-0610-42-05.png.1a8a7e1f44f37d3a729735f886c10f1f.png

376188718_Screenshotfrom2021-11-0609-33-54.png.6b046427258ee62bd801860060d35782.png2101494954_Screenshotfrom2021-11-0610-49-05.png.6d1ae1d7060a7c3588a43814bc18e8b4.png1169980299_Screenshotfrom2021-11-0610-58-58.png.14055bb1bf52368a14adffa4e05181e7.png

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14 hours ago, High Bridge Pottery said:

What I am trying to say is the solubility of gasses in a glaze is a significant source for bubbles and some of that starts to dissolve long before 800c~ when the carbonates are decomposing as the OP is thinking.  If you look at the frit sintering and softening temps below they are all under 800c.

Very well - looks reasonable.

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Good to be back :lol: been a long time since I have done any studio pottery but I am waiting on the sale of a house to go through and set up there. Currently working for Portmeirion Pottery printing their artwork onto flatware which has been fun to learn.

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