Jump to content

Gerstley borate on earthenware

Recommended Posts

Hello, I have bought a bucket of gerstley borate, and have started making some glaze samples.

I want to use GB as a cheaper alternative to calcium borate in my earthenware clay glaze.  I have had serious problems with crawling, using 2 parts clay and 1 part calcium borate also.   

In my glaze samples now with GB I used 1 part kali feldspar, 1 part GB and 2 parts blue clay (grey earthenware), and same with 2 parts GB.  1-1-2 and 1-2-2, feldspar, GB, clay.  And with additions of 0.4 parts FeO2 and 0.2 parts Pb02 (lead) .  I was expecting to see this melt easily and completely, but to my eye the glaze turned out totally matte and barely non porous.

What is the reason for the lack of vitrification or glassyness.  Can anyone give me hints about this?


The white samples have small amounts of tin oxide and the bottom has CuO2

Link to post
Share on other sites

Things melt by composition and Gerstley is just a source of boron but also contains silica, alumina and fluxes . Things melt by composition and boron is an easy way to get the earths geology to melt below cone 10-ish. In a glaze with typical flux ratios of approximately 0.3:0.7, about 0.15  boron (UMF) glazes melt around cone 04.

All of this is likely more than you want to know, so by trial and error Gerstley or fritts may provide a better opportunity to experiment doing some sort of apportioned melt test or line blend if you will in hopes of finding the melt you want.

You could pop this in a glaze calculator to quantify and work towards 0.15 boron. Again these things melt by chemical composition so without the glaze calculator it’s often hard to understand the trends by trial and error.

Link to post
Share on other sites
2 hours ago, AOEYnes said:

0.2 parts Pb02 (lead)

I don't recommend the use of lead whatsoever. It is a danger to everyone involved, from the miners, to the potter and to the end user of the product. Serious and profound health implications with the use of lead. This has been discussed on this forum many times, a link to one good discussion here.

A simple gerstley borate cone 04 - 03 recipe is 55 gerstley borate, 30 kaolin and 15 silica.  This is measured in weight not parts, do you have an accurate scale? What temperature are you firing to and what type of kiln? Do you have access to pyrometric cones or are you going by temperature alone? Your glaze is unmelted either because it doesn't contain enough flux in relation to the silica and alumina amounts or it is underfired. We can't easily run your recipes through glaze calculation programs as it is measured in parts and not by weight so determining which of the reasons it's unmelted isn't straightforward.

Welcome to the forum. 

Edited by Min
added a thought
Link to post
Share on other sites

Ok, thank you.  I am not very accustomed to the way of calculating.  I try to have my recipes as simple and symmetric as possible because my base glaze are built around the same earthenware clay that I use for the bodies.  The clay is arctic marine clay and I glazefire at 1040 C° 

Kaolin is much more refractory, or melt resistant than my marine clay.  Mixing with marine clay should lower the melting point as it melts completely at 1150C°, or?

Ok, I know the mix 2-2-1, GB-clay-kalifeldspar melts at 1260C°, but I cant go higher than 1100 when I use earhenware.  And I want to burn my glazes at 1040.

The reason I now use feldtspar in my glaze is that I have found that it helps with the crazing that would otherwise have occured if I only used Clay and Calcium borate as I used to.   Could the feldtspar be retarding the melting process when I now use it together with GB.

Could a mix of 50/50 gerstley and clay be better?

I use an accurate digital scale and I use a nabtherm electrical oven.

Link to post
Share on other sites
4 hours ago, AOEYnes said:

Could the feldtspar be retarding the melting process when I now use it together with GB.

The feldspar could very well be raising the maturation temperature of the glaze. Kalispar probably wouldn't melt on its own at cone 10. I'm just shooting in the dark here because I have no idea what is in your marine clay, do you have any sort of analysis for it? To bring the maturation temperature of the glaze down I would imagine you would need to increase the gerstley borate and then to help with the crazing try adding silica to the point where the glaze still melts well.  It's going to take a fair bit of testing.

Link to post
Share on other sites
5 hours ago, AOEYnes said:

And I want to burn my glazes at 1040.


1 hour ago, Min said:

The feldspar could very well be raising the maturation temperature of the glaze. Kalispar probably wouldn't melt on its own at cone 10.

I use cone 10 clay as body for most of my work.  
When doing glaze assignments we have to make a plate that shows what happens at the bisque and final glaze firing for each ingredient of the glaze recipe assigned.  We (us students) learn a lot from these little plates of individual ingredents.  

My recollection is that Gerstley Borate (GB) melted to a glaze coating at our bisque temperature -- a tad below 1000 C. We use a mix of GB and nepheline Syenite as the base for a our clear Raku glazes fired at ~1000 C.  They melt well at that temperature.   When I was using earthenware clay in Raku firings there is some crackle on the Raku ware; probably could have been tweaked away if one wanted to do so (I didn't want to).  Worth trying, maybe. 

I haven't seen any feldspar melting at our bisque temperature (~1000 C), bentonite yes, but feldspar no. 


Link to post
Share on other sites
5 hours ago, AOEYnes said:

1100 when I use earhenware.  And I want to burn my glazes at 1040.

So interesting - you are firing between 04 and cone 01. So lowfire glazes may be a way to go or something to try. There are many recipes on glazy.org so this may get you closer. As far as making one by weight with materials we do not know the chemistry of, it will be difficult ........ or if you are lucky an early try in the trial and error bucket works. I don’t have anything productive to say of the trial and error stuff except my suggestion would be to only change one ingredient at a time and test a progressive blend to understand the trend. Once you start to vary two things, it’s very hit and miss. Buy a lottery ticket just for insurance.

Link to post
Share on other sites

I have had pretty good results with 1 part calcium borate and 2 parts clay.   Without any addition of oxides it have a tendency to crawl.  That and the fact that calcium borate is quite expensive made me want to try GB.   This glaze is intended to be used in kitchenwares, bowls, jugs, etc.  Rustic and easy, so the glaze does not need to be crystal clear or super smooth.    

Link to post
Share on other sites

Very different than the general cultural philosophy here where items that come in contact with food are usually fully vitrified and covered with glazes that are durable and as they say locally, food safe. Lead would be a non starter here.  Interesting in that there was only one mine in the western US that mined gerstley. Surprised it’s available and economical.

Edited by Bill Kielb
Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.