High Boron Glaze Question

[algebraist]

A few years back there was a thread on oil spot glazes in which John Britt posted a 2-part (cone 6) combination recipe, a black oil-spot base glaze with a white covering glaze.  The recipe for the covering glaze was:

 

Oil Spot Combo #2 CoverGlaze (apply over #1) Cone 6 (1 - 2 coats)

30g. –Custer Feldspar
30g. – Gerstley Borate
25g.- Silica
5g. – EPK
10g. Zircopax

 

When I take a look at the unity formula for this in my glaze software (Insight), it shows a B2O3 level of 0.580.  That's pretty far outside the usual range (John Hesselberth and Ron Roy, in "Mastering Cone 6 Glazes," suggest a range of between 0 and 0.3 in their appendix on Limit Formulas).

 

So my question is: Does anyone have an idea of what the pitfalls might be to going so far outside usual limits for B2O3?  Would it make the glaze unsuitable for food contact (if, for example, I used the combination of two glazes on the outside of a piece, but this second glaze alone on the inside as a "liner")?

 

One thought is that perhaps the zircopax makes it so refractory that it counteracts the excess of boron, and just works nicely, but I'm just speculating.

 

I will probably try it regardless and see how it goes, but if anyone has any ideas to share, I'd like to hear them.

 

Also, I wonder (and will also try) if there's anything that makes this white covering glaze an especially good partner for the oil spot base, or if a standard white glaze will do just as well.

 

Final thought: My goal is not to achieve the classic oil spot, but white over an oil spot can make a nice speckled pattern, and that's what I'm looking for. 

[Min]

"So my question is: Does anyone have an idea of what the pitfalls might be to going so far outside usual limits for B2O3? Would it make the glaze unsuitable for food contact (if, for example, I used the combination of two glazes on the outside of a piece, but this second glaze alone on the inside as a "liner")?"

 

I’ve always thought of boron as being the ingredient in the recipe that comes with a dual personality. In reasonable amounts it’s great at reducing crazing, melting nicely, making a good gloss etc but if it is way oversupplied it has another side. Large amounts may cause crazing, blisters and crawling, it can also leach out some colourants.

 

All that being said I tried that glaze years ago and it had a good melt with no visible flaws, toilet bowl white. I didn’t use it in the end so I didn’t test for crazing or durability. Just looking at the numbers I wouldn’t be too concerned with it. You could try another white glaze with less boron but I’m not so sure you would get the same spotting. Lots of the really neat looking glazes are outside the “limitsâ€, Hesselberth and Roy’s Waterfall Brown for example has boron at 0.76 (unity). I would just test it for crazing / durability if being used for a liner glaze. (I doubt you will see leaching though)

 

 

By the way, I tried the base glaze with the iron and subbed some cobalt into top glaze and got the same spotting but with blue. kinda neat looking too. (the glaze thickness really makes a difference it the size of the spots).

[High Bridge Pottery]

Although you can predict the chemistry of the glaze, having it layered over another glaze will change that chemistry. I think high B2O3 does lead to leeching and a softer glass but it is worth testing what comes out your kiln, see if it stains or changes colour under attack from acid/base. See if you can scratch it.

 

At a quick look on the other thread it looks to me like the dark iron glaze will probably stay put and the high boron white will do all the moving and 'oil spotting' so another plain white would not give you the look you want.

 

I think the pitfalls of using the high boron is that you get oil spotting 

[MatthewV]

With boron in a ∆6 glaze, I would only be concerned with being under the limit if the glaze is for domestic ware. Somewhere between 0.15 and 0.20 is the minimum to make a strong, glossy glaze. Adding more boron doesn't help the strength or the gloss of the glass formation. It shouldn't hurt it much either.

[David Woodin]

A few years back there was a thread on oil spot glazes in which John Britt posted a 2-part (cone 6) combination recipe, a black oil-spot base glaze with a white covering glaze.  The recipe for the covering glaze was:

 

Oil Spot Combo #2 CoverGlaze (apply over #1) Cone 6 (1 - 2 coats)

 

30g. –Custer Feldspar

30g. – Gerstley Borate

25g.- Silica

5g. – EPK

10g. Zircopax

 

When I take a look at the unity formula for this in my glaze software (Insight), it shows a B2O3 level of 0.580.  That's pretty far outside the usual range (John Hesselberth and Ron Roy, in "Mastering Cone 6 Glazes," suggest a range of between 0 and 0.3 in their appendix on Limit Formulas).

 

So my question is: Does anyone have an idea of what the pitfalls might be to going so far outside usual limits for B2O3?  Would it make the glaze unsuitable for food contact (if, for example, I used the combination of two glazes on the outside of a piece, but this second glaze alone on the inside as a "liner")?

 

One thought is that perhaps the zircopax makes it so refractory that it counteracts the excess of boron, and just works nicely, but I'm just speculating.

 

I will probably try it regardless and see how it goes, but if anyone has any ideas to share, I'd like to hear them.

 

Also, I wonder (and will also try) if there's anything that makes this white covering glaze an especially good partner for the oil spot base, or if a standard white glaze will do just as well.

 

Final thought: My goal is not to achieve the classic oil spot, but white over an oil spot can make a nice speckled pattern, and that's what I'm looking for. 

The food safe limits show .15 to .35, Matrix shows 0 to .3850, Insight shows .3 to .5. If you want food safe  I would stick to the food safe limits.  Other wise test it for your use.

David

[algebraist]

Thank you all so much for sharing your expertise and experiences.   Min, it was especially helpful and encouraging to hear from someone who has tried the actual glaze in question (and thanks, too, for the suggestion about the cobalt).  I will give it a try and see what I see.

[bciskepottery]

Drop an email to John and ask him . . .

 

http://johnbrittpottery.blogspot.com/

[Tim T]

Up to about 15% boron reduces crazing, but go much above that and crazing can return due to the Boron making the glaze brittle.

High amounts of Boron can increase the solubility of the glaze in water.

I do a fair bit of high fired oil spot glazes, and the oil spots definitely come from the base glaze, not the over glaze. The oil spot forms because the iron decomposes and gives off oxygen, which rises to the surface with a glob or iron, and the oil spot is the resultant crystal - long thought to be magnetite but a recent paper testing some old Chinese glazes reckoned it was a form of haematite. The surface glaze has to be fluid enough to part and let the air out, but not so fluid as to heal over again, to get the sort of effect I've seen in pics of John's work.