Jump to content
ISY

Food safety in glazes

Recommended Posts

Hi,

does anyone have any advice on how to ensure a glaze is food safe and doesn't leach anything harmful? I know that it's possible to send samples off to get tested, but for a hobby potter this is not too useful since even if I know that one batch is OK, I can't adequately ensure that I'll meet exactly the same conditions for a subsequent batch and I can't keep sending stuff off for testing, it would get too expensive.

So far I've just avoided the problem by only making ornamental ware, but it would be quite nice to make a few mugs that could be used at home. Are there any glazes that simply don't have anything harmful in them, so that even if they leach they are not going to make anyone ill?!

Edited by ISY
yappystudent likes this

Share this post


Link to post
Share on other sites
8 minutes ago, C.Banks said:

A search of the site unfortunately does not easily bring up this Clay and Glaze faq.

 

That is great, thanks! Lots of info on there!

Share this post


Link to post
Share on other sites

Why can't you have repeatable batches? As long as you're not changing the formula of the glaze, it's very repeatable. Just fire to the same cone each time.

Ultimately, you need to educate yourself on glaze materials and their safety, as well as what makes a good glaze. If possible, take a glaze formulation class somewhere, or buy some of the more recent glaze formulation books like Mastering Cone 6 Glazes, or John Britt's recent book.

Pres and ISY like this

Share this post


Link to post
Share on other sites

As Neil says, if you are using a formula/recipe and are accurate with your balances you should have no trouble in sustaining repeated results from each firing, if you are using the same cones for each firing. I use a triple beam balance with large container to weigh my bulk materials, have my formulas in a spread sheet with formulas set so that I have 5000 and 7500 gram batches. I use an erasable marker over the plastic sleeve in my binder to mark each ingredient as it goes into the bucket. Erase off when done til next time. In the end consistency comes with practice and organization.

 

best,

Pres

Marcia Selsor, Babs and ISY like this

Share this post


Link to post
Share on other sites
 
Get the book: MASTERING CONE 6 GLAZES and read it carefully, not just for the recipes and pretty pictures, but for the insights on glaze stability. 
The book is available from various sources including iBooks and directly from the authors.  
Spend time on
 the book website: http://www.masteringglazes.com/ 
 authors' websites:
John Hesselberth: http://frogpondpottery.com/
Ron Roy: http://www.ronroy.net/
 
 
lt
ISY likes this

Share this post


Link to post
Share on other sites
6 hours ago, ISY said:

Are there any glazes that simply don't have anything harmful in them, so that even if they leach they are not going to make anyone ill?!

Yes. This is why many potters use liner glazes inside pots. Balanced glaze with no colouring oxides and no harmful ingredients fired to maturity. Clay (kaolin or ball clays), silica, feldspar, fluxes such as calcium carbonate, boron, talc, dolomite in a clear glaze are not going to be a problem. If you make your own glaze you know what you are putting into it, if you use commercial glaze look up the MSDS or SDS (Safety Data Sheet) for what's in it, if it doesn't tell you then contact the glaze manufacturer and ask if you are still concerned.

ISY likes this

Share this post


Link to post
Share on other sites

I use two liner glazes in all my wares that liquids go into-they are very neutral and all the ingredients are safe.I would eat them all myself.

I make my own glazes-but am firing to cone 10.

ISY likes this

Share this post


Link to post
Share on other sites

Thanks Neil and Pres, I had just assumed that I couldn't re-create conditions adequately well to have comparable batches. My kiln is also still a bit temperamental, but I'm hoping to get that sorted as soon as I can. So maybe this isn't a totally unachievable thing for me after all!

Thank you also Pres, for the suggestions re. mixing glazes, I really like the erasable marker idea, going to try that.

 

Share this post


Link to post
Share on other sites
3 hours ago, Magnolia Mud Research said:
 
Get the book: MASTERING CONE 6 GLAZES and read it carefully, not just for the recipes and pretty pictures, but for the insights on glaze stability. 
The book is available from various sources including iBooks and directly from the authors.  
Spend time on
 the book website: http://www.masteringglazes.com/ 
 authors' websites:
John Hesselberth: http://frogpondpottery.com/
Ron Roy: http://www.ronroy.net/

Thank you so much for those links! I'd actually looked at that book on Amazon a little while ago because I saw it recommended, but the price tag of over £200 was a little off-putting. I'll see if it's a bit more affordable directly from the authors.

Share this post


Link to post
Share on other sites
1 hour ago, Min said:

Yes. This is why many potters use liner glazes inside pots. Balanced glaze with no colouring oxides and no harmful ingredients fired to maturity. Clay (kaolin or ball clays), silica, feldspar, fluxes such as calcium carbonate, boron, talc, dolomite in a clear glaze are not going to be a problem. If you make your own glaze you know what you are putting into it, if you use commercial glaze look up the MSDS or SDS (Safety Data Sheet) for what's in it, if it doesn't tell you then contact the glaze manufacturer and ask if you are still concerned.

Min and Mark C., I had assumed that if I use for example a transparent glaze over the top of another glaze they would blend/melt together in the firing process? I can see I have a lot to learn about glaze chemistry. It was never my strongest subject at school, so it's going to be a bit of an adventure I think...

Is there any definite list of glaze ingredients and toxidity that I could refer to? As it is I'm going with the label on the bags the stuff came in but while it does say if something is really toxic (Nickel comes to mind,  which has a reassuring little skull on it!), but it would be useful to have a list for less toxic materials that might still be moderately harmful if they leached out of a glaze. I don't think I'd ever attempt to drink out of something glazed with a Barium glaze, but there are so many glaze materials that I just don't know well enough to be sure. 

Edited by ISY
typos

Share this post


Link to post
Share on other sites

Related question to all you knowledgeable folks - I know that manganese can be a problem, and I am not using it for anything food related and am also careful not to spend time around the kiln when firing. However, if I have a decorative item that is definitely never going to be used for food, that is fired  with just manganese (sponged on, I really like the look of that), would this pose a problem if the item was occasionally handled? Nobody is going to lick it, or anything, but could toxic substances be absorbed through the skin for example? Thanks. 

Share this post


Link to post
Share on other sites

Getting away from buying expensive new books for a second: You might try looking up some US supplier websites like Georgie's Ceramics website and other places like The Big Ceramic Store, and there must be a few other good ones. I know they have some printable charts and a lot of info there relating to their products and they do sell a lot of raw ingredients for mixing glazes. I have a small collection of old ceramic production books that I picked up second hand, you might spend an evening perusing Amazon or Ebay searching out battered but useful tomes like the ones I have from used bookstores. 

Share this post


Link to post
Share on other sites
an addendum to my prior post:
alternative sources for:
 Mastering Cone 6 Glazes
by: John Hesselberth & Ron Roy
Publication Date: July 11, 2013
Book Size: 6" x 9"
Pages: 240
Binding: Perfect Bound
$24.95
https://www.thebookpatch.com/BookStore/mastering-cone-6-glazes/d2bea83c-2c34-4ed0-8a00-a6f12113515d 

https://itunes.apple.com/us/book/mastering-cone-6-glazes/id573583135?mt=11 
$19.99
Available on iPhone, iPad, iPod touch, and Mac.
Category: Art & Architecture
Published: Oct 25, 2012
Publisher: Glaze Master Press
Seller: John Hesselberth
Print Length: 168 Pages
Language: English
 

Share this post


Link to post
Share on other sites

Min - thank you so much for those links! Time and again I am reminded that you really need to go back to the data and how/where it was collected before making a determination about things. For example, I've never quite understood the relative risk of using manganese dioxide in a glaze, and this paragraph from the longer version is quite illuminating:  

An example of ceramic glaze (and body) folklore are the reports surrounding the use of manganese (Mn) and its link to Parkinson’s Disease. Mn in a soluble form is a neurotoxin, but this data was generated from the steel industry and the conditions of exposure are dramatically different due to the vaporization of Mn during steel production (and the use of a furnace open to the local environment, that is, without a stack). Inhalation of these vapors creates a readily available reactive species for adverse interactions with the body (and internally through the lungs and mucus membranes). Even small amounts can be readily absorbed into the bloodstream. The oxides of manganese, MnO and MnO2, are both insoluble in water and do not pose a threat. The Mn is not soluble and therefore not available, cannot be absorbed through the skin or even an open cut, and even if ingested, cannot dissolve into the blood stream. Mn obtained from a salt (chloride, nitrate, or sulfate) is soluble, so if the source of Mn is a salt, exercise appropriate care.

Magnolia Mud Research and ISY like this

Share this post


Link to post
Share on other sites

I suggest you use a very simple liner glaze for interiors-one that has say 4 simple ingredients that are all safe-no colorants are best.Just a white or clear.

ISY likes this

Share this post


Link to post
Share on other sites
43 minutes ago, Mark C. said:

I suggest you use a very simple liner glaze for interiors-one that has say 4 simple ingredients that are all safe-no colorants are best.Just a white or clear.

I'll try that, thanks. I have a very  basic transparent glaze that I bought rather than mixed myself so I'll check what ingredients it has. Being transparent it's hopefully going to be OK. So would I just use my normal glaze, then add a 2nd layer with the transparent glaze? Or do I double-fire, once at a lower temp for the 'questionable' glaze, and then again at a higher temp with the transparent glaze? Thanks for all the help and advice, I appreciate it. 

Share this post


Link to post
Share on other sites

No need to twice fire

tell us again your firing temp?cone 08? cone 6? cone 10?

Liner glazes are for interiors-glaze them first then apply outside glaze-no need to overthink this to much.

 

Share this post


Link to post
Share on other sites
2 hours ago, Mark C. said:

No need to twice fire

tell us again your firing temp?cone 08? cone 6? cone 10?

Liner glazes are for interiors-glaze them first then apply outside glaze-no need to overthink this to much.

 

I'm currently firing to cone 8 but also plan to occasionally fire cone 6. 

Sorry, I probably am overthinking this! So I just apply the liner over the glaze  I want to use on the interior, then apply glaze as wanted on the exterior (but without additional liner), and then fire to temp? 

Share this post


Link to post
Share on other sites

I always happy the inside glaze 1st and let it dry then outside. There are a few reasons for this but the big one is you do not want to over handle the outside glaze and chip or wear it off.

Another reason is you want the exterior to not have any runs on top of it as it looks sloppy

We always glaze the interiors first-no exceptions

ISY likes this

Share this post


Link to post
Share on other sites

no, liner glaze alone in interior of pot.

decorative glaze outside, I put liner down outside, where lips touch pot if I have a questionable glaze on outside.

ISY likes this

Share this post


Link to post
Share on other sites

I also sponge the lips of questionable glaze-I did not want to get to detailed for general use.But since we are on it.

I have a barium outer glaze on some glasses forms that is little used -Then I still coat inside liner than dip outer glaze up to lip and let dry. Then power sponge(whole other story ) (sponge works fine) the lips and dip with liner where your lip touches pot. Only have one glaze where the is an issue and hardly use it much.

Babs and ISY like this

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×

Important Information

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