Adding A Touch Of Gerstley Borate?

[PottaFella]

Got a feeling every potter goes through the phase of experimenting with Gerstley Borate? Anyways, it's new to me in the UK and hoping the 1kg bag I've invested in will add some interest to some of my glazes which my wife complains are rather flat.

 

I'm firing stoneware at cone 71/2, ideally I'd stick with this schedule.

 

Can someone give me an idea of how much GB to experiment with adding to a glaze to get a noticeable effect? 10%?

 

Also, is it likely to lower the melt point of the glaze? Is that what it does?

 

Sorry, I know this post is way too vague but hope some answers will start to rein it in.

[Callie Beller Diesel]

https://digitalfire.com/4sight/material/gerstley_borate_806.html

 

This article answers all of your posted questions.

Just to clarify a bias though: Tony Hanson doesn't like the stuff, because it can make a shiny glaze that is actually soft if you don't have enough silica and alumina, and it does tend to pinhole things. But it makes your glazes pretty, and easy to use because they don't settle out, so a lot of people disagree with him. But this about sums up the pros and cons of Gerstley Borate.

[High Bridge Pottery]

Adding 10% GB may just make your glazes glossier and a bit more fluid. Here is a test tile I did using only GB as the flux. Going up increases in Kaolin and to the right increases in Silica. There's a certain point that you get some depth with 'boron clouding'. I remember it being related to how much silica is in the recipe too. Cone10.

 

Do you have any rutile/titanium/tin? Have you tried layering glazes?

 

[Marcia Selsor]

I used 10% Gerstley Borate as a starting point for lowering ^10 glazes to ^6. The lower temperature kept the glazes original look for most of the time. some seeking may be needed .

So adding 10% to a stoneware glazes and firing at the same temperature will definitely make it shinier and possibly run.

 

 

Marcia

[Magnolia Mud Research]

My approach would depend on whether I was trying to create from scratch a glaze that included a significant amount of GB or if I was trying to modify an existing glaze by  adding or substituting GB to the glaze.
 

If it were a from scratch point of view, I would use the Currie Grid approach illustrated by Joel.  This would give me quick look at the landscape and allow me to narrow down the region I wanted to explore with narrow range line blends.
 

If it were the modify an existing glaze to just add GB, I would start with a set of line blends between 100 % existing glaze and 100 % GB.  I would use 9 steps. Because I would mix a big batch of the 100% ends and volumetrically mix to form the middle and then split the halves into halves and the half-halves into half, etc., or just measure by thimble measures to produce an even spread of ratio of EG (existing glaze) and GB. 
 

The really difficult decisions are what properties of the new glaze are you going to evaluate and how will you measure the results.  My inclinations drive me to just do a coarse screening experiment and after the fired test tiles are available follow what looks exciting. 
 

I fire at cone 10, and GB produces a nice clear glaze on stoneware. I have not tried it on porcelain.  I have no data or speculation on the leaching or fit since I was experimenting for decorative effects on sculptural objects where leaching or fit was not an important constraint.   I do recall that GB can show up in a wide range of glazes from Raku to cone 10.  It is a reasonable alternate source of calcium and boron oxides without using frits.  It does change the rheology of the glaze slurry relative to glazes without GB, but you will know that going in and can avoid the 'knee-########' responses to rheology fluctuations.
 
LT

[neilestrick]

My question is why do you feel the need to use Gerstley? If you've already got a good source of boron, whether it be a frit or a mineral, then you don't need Gerstley. It's just a source of boron, used as a flux and to some degree a glass former. But it's by no means some magical ingredient. It's a cheap source of non-soluble boron, but beyond that there's no benefit to using it. In fact, like the Digitalfire article say, frits are a better source of boron because they don't have the problems that Gerstley has.