Jump to content

bright blue food safe glaze


Recommended Posts

I'm a new potter, and looking into making my own glaze. I'm concerned about how to make such a bright glaze on a stoneware body, most likely firing to cone 7/8 and that is this bright in colour AND most importantly food safe. I'm learning the chemistry behind it but ingredient suggestions might be helpful or just any help. 

 

Second sale april 2020 photo -1.jpg

Link to post
Share on other sites

Hi Imogen and welcome to the forum.

Cone 7/8 is an unusual cone to fire to, is this the range in which your clay matures or are you using one that is listed as having a broad firing range or is it a cone 10 clay? For functional work you want the clay to be vitrified.

Looks like a cobalt blue satin or semi matte glaze. Using a clear or white gloss liner glaze is a good idea if you want to avoid the possibility of the cobalt leaching and cutlery marking with a matte glaze. You can do a couple home tests to rule out leaching glazes, there is also the option of having glazes lab tested. Glazy has quite a number of recipes, fill in your search terms on the left side of the screen. Cone 7/8 might not have many though.

Having plenty of both silica and alumina in the glaze, firing it to maturity, not overloading most colouring oxides or fluxes and testing the glazes before putting them to use on functional pots are all important. Mastering Cone 6 Glazes is a great book for a fairly basic intro to making glazes and well worth reading if you are wishing to learn more about what makes a durable glaze. 

Link to post
Share on other sites
7 hours ago, ImogenB said:

I'm a new potter, and looking into making my own glaze. I'm concerned about how to make such a bright glaze on a stoneware body, most likely firing to cone 7/8 and that is this bright in colour AND most importantly food safe. I'm learning the chemistry behind it but ingredient suggestions might be helpful or just any help. 

 

Second sale april 2020 photo -1.jpg

That is most likely a stain or a very saturated cobalt. Most saturated cobalts are dark blue, I have never seen such a light blue like that with cobalt.

 

Look at these mason stains below.

https://www.theceramicshop.com/product/8693/delft-mason-stain-6320-1-lb/

https://www.theceramicshop.com/product/8703/copen-6368-mason-stain-1-lb/

https://www.theceramicshop.com/product/8682/vivid-blue-6306-1-4lb/

 

Note: Stain are usually between 3-10% in glazes. If you want to make (say a 5000g batch) you are better off getting 1 lb of stain, many people (myself included) often get 1/4 lb and it's a few grams off, and there is not enough for a batch.

Link to post
Share on other sites
14 hours ago, Min said:

Hi Imogen and welcome to the forum.

Cone 7/8 is an unusual cone to fire to, is this the range in which your clay matures or are you using one that is listed as having a broad firing range or is it a cone 10 clay? For functional work you want the clay to be vitrified.

Looks like a cobalt blue satin or semi matte glaze. Using a clear or white gloss liner glaze is a good idea if you want to avoid the possibility of the cobalt leaching and cutlery marking with a matte glaze. You can do a couple home tests to rule out leaching glazes, there is also the option of having glazes lab tested. Glazy has quite a number of recipes, fill in your search terms on the left side of the screen. Cone 7/8 might not have many though.

Having plenty of both silica and alumina in the glaze, firing it to maturity, not overloading most colouring oxides or fluxes and testing the glazes before putting them to use on functional pots are all important. Mastering Cone 6 Glazes is a great book for a fairly basic intro to making glazes and well worth reading if you are wishing to learn more about what makes a durable glaze. 

Hi Min,

Thanks for your helpful response.

I am firing to cone 8 to help the longevity of my kiln elements, my clay vitrifies from cone 6. The liner glaze is a good idea, but i am still worried i'd need excessive cobalt to achieve this and therefore would still be not food safe.

I have just ordered the book, thank you for the suggestion. 

 

 

Link to post
Share on other sites
14 hours ago, Brandon Franks said:

That is most likely a stain or a very saturated cobalt. Most saturated cobalts are dark blue, I have never seen such a light blue like that with cobalt.

 

Look at these mason stains below.

https://www.theceramicshop.com/product/8693/delft-mason-stain-6320-1-lb/

https://www.theceramicshop.com/product/8703/copen-6368-mason-stain-1-lb/

https://www.theceramicshop.com/product/8682/vivid-blue-6306-1-4lb/

 

Note: Stain are usually between 3-10% in glazes. If you want to make (say a 5000g batch) you are better off getting 1 lb of stain, many people (myself included) often get 1/4 lb and it's a few grams off, and there is not enough for a batch.

Brandon,

Thanks for your reply.

I was wondering if it was stain. I'm yet to test one out, I shall get experimenting.

This is the work of Helen Levi, I would think she tests her work to see if it's food safe, so i'm sure it can't be saturated cobalt as that wouldn't be safe.

And thanks for the extra tip about amounts, i'll make sure to larger amount when ordering.

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

Brandon,

Thanks for your reply.

I was wondering if it was stain. I'm yet to test one out, I shall get experimenting.

This is the work of Helen Levi, I would think she tests her work to see if it's food safe, so i'm sure it can't be saturated cobalt as that wouldn't be safe.

And thanks for the extra tip about amounts, i'll make sure to larger amount when ordering.

Here, I just came across this, looks nearly identical.

 

I saw it and immediately thought of your posting. Give it a whirl and let us know how it comes out.

https://glazy.org/recipes/2467

 

Also, You can probably squeeze this in a cone 8 load. If you want, I can make a test of it, Im getting rid of my last bit of HF clay, and don't have enough glaze for all the pieces

 

Edited by Brandon Franks
Link to post
Share on other sites
16 minutes ago, Brandon Franks said:

I saw it and immediately thought of your posting. Give it a whirl and let us know how it comes out.

https://glazy.org/recipes/2467

Would not recommend using a glaze with 40% barium carb, especially for functional work. https://digitalfire.com/4sight/hazards/ceramic_hazard_barium_in_materials_and_fired_glazes_26.html

@ImogenB, not sure I'm following you when you say "I am firing to cone 8 to help the longevity of my kiln elements, my clay vitrifies from cone 6." I'm guessing this a clay that is advertised as having a wide firing range? 

Edited by Min
Link to post
Share on other sites
19 minutes ago, Hulk said:

Nice color!

...however, "Significant Flaw: potentially toxic"

Barium, just not seeing a need, excepting for a smidge in scummy clays...

It also says copper carbonate, I'm guessing to make barium copper silicate or "han blue"?  Copper and barium that recipe would be very sensitive to any acid attack, probably straight tap water even

 

Link to post
Share on other sites
20 hours ago, Min said:

Would not recommend using a glaze with 40% barium carb, especially for functional work. https://digitalfire.com/4sight/hazards/ceramic_hazard_barium_in_materials_and_fired_glazes_26.html

@ImogenB, not sure I'm following you when you say "I am firing to cone 8 to help the longevity of my kiln elements, my clay vitrifies from cone 6." I'm guessing this a clay that is advertised as having a wide firing range? 

Yes I'm not sure that Glaze is suitable, I think i'll avoid using Barium.

Yes it does have a broad firing range. I have an electric kiln, so it'd be an oxidise firing.

I'm from the UK so using Fahrenheit isn't natural to me, so i'll use Celsius, but I usually fire to 1260oc.

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

Yes it does have a broad firing range. I have an electric kiln, so it'd be an oxidise firing.

I'm from the UK so using Fahrenheit isn't natural to me, so i'll use Celsius, but I usually fire to 1260oc.

I thought that might be the case. Just a heads up about broad firing range claybodies, what is mature at the top of the firing range can't also be mature at the lower end of the range. A clay rated to go to cone 10 for example won't be mature at cone 6. I get that you want to extend the life of your elements but I'ld run some absorption tests on the clay to ensure that it won't leak at cone 7/8. If you haven't already done this there are directions on how to do this about 2/3rds the way down this page.

Link to post
Share on other sites
  • 3 months later...

Hi can anyone offer any further advice?

I made this glaze and fired it to 1280oc. It is changing colour when i put in lemon over night to test for leaching, as you can see there are white speckles where the cobalt is no longer.

The recipe is: 

Potash Feldspar 30g

Quartz 36g

Whiting 23g

China Clay 11g

Cobalt Carbonate 1g

 

Can anyone help to why this is still leaching? 

111-1.jpg

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.

Guest
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.