Food Safety

[Guest Sherman]

Every other question we get here at CAD (well, maybe every third) is whether or not a glaze or a particular method is food safe. Our answer is always either "no," or we refer them to a lab that performs leach tests (there are several). It's really the only way to verify what your glaze, in your kiln, in your studio is really producing. There are too many variables between different studios, specific materials, mixing and storage methods (the list is long) that can't be accounted for between different studios.

 

So, mix and fire any glaze you are interested in yourself, and have those tested by a lab. Here is one such lab that has online instructions on how to prepare and submit materials for testing:

Brandywine Science Center http://www.bsclab.co...lab_pottery.htm

 

Happy testing!

 

P.S. Anyone else know of other labs with good testing instructions online?

[mudstuffing]

We recently had this discussion within the EtsyMudTeam. Here is some more info/testing sources (originally posted by Jen Wolfe):

 

Alfred Analytical Laboratory

http://www.frogpondpottery.com/glazetest.html

Phone: 607-478-8074

Location: Alfred Station, NY, USA

Will do leach testing for extractable metals in fired glazes. Contact Dr. Roland Hale, Director, Alfred Analytical Laboratory, 4964 Kenyon Road, Alfred Station, NY 14803

 

 

Informative links ...

 

Massive list of testing labs. Some non-USA labs are listed.

http://digitalfire.com/services/database.php?list=labs

 

Calm and clear article on glaze testing from Big Ceramic Store. It will help you understand what to test for.

http://www.bigceramicstore.com/information/Tip53.htm

[Nannyflun]

Every other question we get here at CAD (well, maybe every third) is whether or not a glaze or a particular method is food safe. Our answer is always either "no," or we refer them to a lab that performs leach tests (there are several). It's really the only way to verify what your glaze, in your kiln, in your studio is really producing. There are too many variables between different studios, specific materials, mixing and storage methods (the list is long) that can't be accounted for between different studios.

 

So, mix and fire any glaze you are interested in yourself, and have those tested by a lab. Here is one such lab that has online instructions on how to prepare and submit materials for testing:

Brandywine Science Center http://www.bsclab.co...lab_pottery.htm

 

Happy testing!

 

P.S. Anyone else know of other labs with good testing instructions online?

 

[Nannyflun]

In the UK try CERAM at Stoke-on-Trent www.ceram.com

[Guest JBaymore]

"Food safe" is a pretty tricky term.

 

For those in the USA or selling in the USA....... here are the only two oxides "formally" regulated by law: Lead and cadmium compounds.

 

See here:

 

http://www.fda.gov/I...l/ucm074516.htm

 

 

http://www.fda.gov/I...l/ucm074515.htm

 

 

ASTM methodology (for the above):

 

http://www.astm.or1g...ndards/C738.htm

 

(you have to purchase it)

 

 

ASTM Ceramic Materials and Products Testing Standards:

 

http://www.astm.org/...-standards.html

 

 

best,

 

...................john

 

PS: California has standards that are different and more restrictive than these.

[Dinorah]

Is Barium Carbonate still considered toxic if fired to cone 10 temperatures?

[Idaho Potter]

Rather than a reply, this is another question. If you purchase commercial glazes that the producer's (i.e., Laguna or Clay Art) note as "food safe" can you believe them? Do you think they've actually done the tests or had the tests done at one of the above labs?

 

Shirley

[Guest JBaymore]

Rather than a reply, this is another question. If you purchase commercial glazes that the producer's (i.e., Laguna or Clay Art) note as "food safe" can you believe them? Do you think they've actually done the tests or had the tests done at one of the above labs?

 

Shirley,

 

I'd personally take the label with a large grain of salt.

 

Why? Liability. Both legal and moral.

 

The glaze manufacturers/suppliers do not indemnify the users of their products. What that means is that if there ever were a case of liability attibuted to the glaze (small possibility ... but it is there), it would be up to you to defend yourself in that case. Bringing up the glaze manufacturer's written statement of "food safe" would be one aspect of mounting a defense that YOUR lawyer would attempt to use in your defense. Defending such a case will be costly (that it why you carry produict liability insurance).

 

If it turned out that the claims on that glaze were actually untrue, YOU would have to take the glaze manufacturer to court to recover the damages that a plaintiff had won against you. With all of the costs that would entail up front for the legal work. And it is likely that the glaze manufacturer hjas a larger legal budget than you might have....and "he/she who hired the best lawyer wins".

 

In the moral department, you are betting YOUR reputation and integrity (and your customer's health and safety) on a simple printed statement by a supplier.

 

Personally I'd have the manufacturer's glaze testing by a lab applied to the body I was using and fired the way I fire. Then I'd have done my own "due dilligence" on the matter. How you apply and fire the product and the underlying clay body and any slips (or crossing ogf glazes) WILL affect its performance.

 

I had an experience with one of the major manufacturers of commercial ceramics glazes and such that was quite "instructive" of this. I'll share it here.

 

I wanted to use a particular product of theirs on my functional tablewares. The "catalog information" on the general line of those items (broad range of colors) said lead free. I worried about this claim because of the particular color and my technical understanding of how this color was produced... which most typically involved the use of lead. I was hoping that they had cleverly found another way (alchemy?).

 

I called the manufacturer's main number and asked to talk to a "technical person" to attempt to verify this information and get more details. I got someone who was supposed to be from their "technical depatrtment", and they assured me the item was in fact lead free. However because I have a very heavy technical background myself, and I was asking detailed questions, it was clear to me that this person actually had LITTLE technical understanding. So I insisted on talking to someone else "higher up". I got shuffled to the next level of "technical person". Same assurance and same feeling of "this person does not know as much as I do about ceramic chemistry".

 

I finally asked if they has a ceramic enginer involved in their technical department. The person said, "yes". I asked to talk to that person. When I talked to THIS person it was clear that they actually were very versed in ceramic chemistry. It took less than a minutte to verify from this person that the particular item that I wanted to use out of the line DID in fact have lead in it.

 

I only finally got to this infomation because I had the heavy technical background myself. The "average" ceramist likely would not have reached that "end stage" answer. Oh........ last time I checked, the catalog listing for this line of products STILL says "lead free".

 

If a manufacturer is claiming "food safe" on a product you'd like to use, call them and ask them to "back that claim up" in writing with representative test data. Then test it yourself.

 

This is an excerpt (Fair Use) from the Consumer Product Safety Commission's paper on craft media: http://www.www.cpsc.gov/cpscpub/pubs/5015.pdf

 

----------------------------------------------------

 

"Read the product label. When possible, choose the safest materials available (e.g., those with few or nocautionary/warning labels.

Label lead- and cadmium-containing pieces with phrasesuch as “Contains Lead, Not for Food Use†or “Contains Lead, For Decoration Only.†Consider designing or puncturing holes in utilitarian objects to discourage use with food/beverages. If there is even a slight chance that your pottery could be used for food, you should have it tested to meet FDA or state standards if you sell it, but also if you just give pieces to family and friends. The liability remains even if you do not sell pottery.

 

 

 

A glaze labeled “food safe†does not mean that it is leadfree, rather it means that if fired and applied properly it will not leach lead or cadmium at concentrations above those allowed by the Food and Drug Administration (FDA) into food or beverages.

 

 

 

Do not mix different glazes together because this disrupts the balance of ingredients and could make a “food safe†glaze into an unsafe product.

 

 

 

Consider testing all finished ware to ensure that it does not leach potentially toxic metals or lead."

 

-----------------------------------------------------------------

 

 

The red highlights are mine.

 

 

 

best,

 

..............john

[neilestrick]

Do not mix different glazes together because this disrupts the balance of ingredients and could make a “food safe†glaze into an unsafe product.

This is the one that most inexperienced potters don't understand. Once you layer two glazes, the chemistry has completely changed, and the safety labels don't apply any more. I often have customers ask me if they can just put a clear glaze over their non-food-safe glaze to make it food safe. No! Glazes are not stiff like underglazes, and cannot be covered over by another glaze. When you layer, they mix.

People also need to remember that there are 2 different safety labels on their glazes. The ASTM D-4236 only refers to the safe handling of the material in the form in which you purchased it. If you purchased moist clay that is labeled non-toxic, that means it is only non-toxic in the most form. Once the clay dries the label no longer applies, and in fact is no longer non-toxic due to respirable silica. The other label applies to the safety of the material once it is fired, such as "food safe". The two labels have nothing to do with each other. A glaze can be 'toxic' and 'food safe', or 'non-toxic' and 'not safe for pieces that will come into contact with food'.

The ASTM safety label may also say that the material is only safe for people above a certain age, typically age 13 or so. This is because young children's bodies cannot safely take in as much toxic material as an adult can. Elementary school teachers need to be careful about what glazes they purchase for use in their studio, a they may only be appropriate for older children. Many blue glazes actually fall into this category due to the cobalt in them. Any glaze with cobalt should also carry the California Prop 65 label, which states that the product 'includes an ingredient that is know to the state of California to be toxic'.

[Kohaku]

Do not mix different glazes together because this disrupts the balance of ingredients and could make a “food safe†glaze into an unsafe product.

 

 

This is an interesting point... and I have a related query.

 

For my cone 6 work, I've been using the guidelines published in Hesselberth and Roy (Mastering Cone 6 Glazes). However, I've been curious as to the implications of using a glaze that is technically not within the 'safe' parameters IF you use a reliable liner glaze on the interior.

 

As an example, I've been using a Lakeside Pottery recipe called 'Weathered Bronze Green' on the exteriors of some sculptural work (gives a nice, Reitz Green'esque patina). This glaze is pretty front-loaded with copper carb- and while it has tested pretty stable, rates of copper leaching are higher than I'd like for functional ware.

 

Is there any reason not to put this on the exterior of something like a mug, however? Given that there's an internal liner glaze, shouldn't the character of the external glaze be irrelevant (assuming it doesn't craze or suffer from durability issues?).

 

Weathered Bronze Green

 

Strontium Carbonate 20

Nepheline Syenite 60

Ball Clay 10

Silica 9

Lithium Carbonate 1

copper carb 5

titanium ox 5

[Guest JBaymore]

Is there any reason not to put this on the exterior of something like a mug, however? Given that there's an internal liner glaze, shouldn't the character of the external glaze be irrelevant (assuming it doesn't craze or suffer from durability issues?).

 

At what likely would be considered a total "technicality", a person's lips MIGHT pick up a tiny, teeny, weenie, trace of anything that might leach out of the exterior glaze in the lip contact area on the outside of the form. But that is likely not an issue. Probably in the "how many angels can dance on the head of a pin" department.

 

However saying that, you should note that the use of cadmium bearing onglaze decoration on the exterior surface of forms has FDA restrictions on how close to the lip of a drinking velssel it is placed. So the concept in general IS looked at in the "legal" and toxicology world.

 

best,

 

.................john

[oldlady]

OK john,

 

 

 

this has been another pet peeve of mine and i am guilty of neglecting the reality of what i do by thinking of the "look" of something, not it's safety.

 

i recently did an inadvertent test mentioned in the "Mastering cone 6 glazes" book by putting some leftover pickled beets in a bowl.i had made it years ago and covered it in a blue glaze i have used often. i used the beets up 4 days later and put the bowl into the dishwasher. when it came out the blue glaze had gone purplish in the bottom of the bowl to the depth of where the beets had been. this bowl was made from a clay i no longer use (because of its color) but i had always trusted the glaze because it came from a classroom situation where everyone used it for anything. there was no testing done in the school and of course i should have done my own. live and learn. obviously, i am not using it again.

 

now i am wondering about the purchased bright red glaze i used on sushi bowls i plan to take to a sale next week. red equals cadmium, doesn't it? cadmium equals unsafe, doesn't it??????

[Guest JBaymore]

 

now i am wondering about the purchased bright red glaze i used on sushi bowls i plan to take to a sale next week. red equals cadmium, doesn't it? cadmium equals unsafe, doesn't it??????

 

Blood red is not ALWAYS from a cadmium source.... but it is often so. The famous "Kutani Red" of Japanese low fire overglaze enamels is a lead and iron reaction.

 

What cone range is the glaze fired at? What did the manufacturer's info say about it?

 

If it is a literal blood red at cone 6 to 10 range it is likely either a reduced copper red (fired in a fuel-burning kiln or using localized in-glaze reduction using very fine silicon carbide powder fired in oxidation) or it contains a cadmium bearing compound. Because cadmiunm reds are fugitive in both reduction and at high temperatures.... it likely is a specacial colorant if this is either of those.

 

Likely if it is high fire, it contains an excapsulated cadmium compound stain. The encapsulated stains are a relatively new additions to the ceramic colorant spectrum. The idea is that the TINY particle of cadmium stain is encased in a little bubble of zirconium silicate glass, completely enclosing it. The melting point of that zirconium silicate "shell" is much higher than the cone 10 range that most high fire potters fire to, so the little "bubbles" are suspended in the fluxed silica glass matrix, scattering the light and giving you the blood red color without the potential leaching issues not the potters handling issues.

 

This is what is usually known as a "suspension color" as opposed to a disolved color. Suspension colors get their character because of tinly little bits of "colored stuff" floating in the glass matrix.

 

While the encapsulated stains help to maintain fuguitive colors like blood red at high firing ranges, and to reduce the handling risks, and reduce the potential leaching risks, they do not absolve the POTTER from following the LAWS from the US FDA and the State of California in regards to how the products must be tested and the approprioate recordkeeping. Also in the case of cadmium compound bearing glazes, WHERE the cadmium bearing glaze may be used.

 

If it is a low fire glaze, like cone 04 range... it very likely has either lead compounds (to get the red in other cheaper ways) or it contains cadmium. This cadmium compound could be in the encapsulated stain department..... OR it could be something like cadmium sulphide. If it is cadmium supphide.... you have some serious handling considerations to look at in the studio.

 

Chrome tim pinks work in the lower ranges........... but they usually are not "blood red". Glold chloride almost blood reds are possible in the very low ranges (I use them) but they are not common and as you might imagine are expensive to produce.

 

I'd call the supplier / manufacturer and get the "inside dope" on what is in the glaze (in a general sense..... they won;t divulge the proprietary formula/recipe). If they won't tell you anything, get an MSDS sheet for the glaze (they HAVE to give that to you) and that MIGHT have some clues as to what is in there.

 

If in doubt.... test.

 

best,

 

...............john

[Guest JBaymore]

i recently did an inadvertent test mentioned in the "Mastering cone 6 glazes" book by putting some leftover pickled beets in a bowl.i had made it years ago and covered it in a blue glaze i have used often. i used the beets up 4 days later and put the bowl into the dishwasher. when it came out the blue glaze had gone purplish in the bottom of the bowl to the depth of where the beets had been. this bowl was made from a clay i no longer use (because of its color) but i had always trusted the glaze because it came from a classroom situation where everyone used it for anything. there was no testing done in the school and of course i should have done my own.

 

You know what they says about ASSumptions. ;)src="http://ceramicartsdaily.org/community/public/style_emoticons/default/wink.gif">

 

Pop the recipe up here and I'll plug it in Insight and take a look.

 

best,

 

...............john

[bciskepottery]

A quick test would be the lemon test from Mastering Cone 6 Glazes . . . put a wedge of lemon on one of the red platters, let it sit for a few hours or overnight, and then see if there is any discoloration. If you have discoloration, then it would appear the glaze is not durable/stable. If it passes the lemon test, then consider sending a piece to a testing lab.

 

Another quick test you can do in the studio is a vinegar test. Fire a test tile with the glaze, insert the test tile into a glass of vinegar so half the test tile is submerged, let sit for 3 days, wash and dry and examine for discoloration.

[oldlady]

john,

 

thank you for your reply. i fire at cone 6. here is the recipe for the blue glaze that turned pink after having pickled beets in the bowl and followed by washing in a dishwasher.

 

LAKE ANNE BLUE CONE 6

 

custer 46.2

dolomite 14.2

silica 18

zinc oxide 5

epk 5.2

gerstley borate 11.4

 

bentonite 2

cobalt oxide 2.5

black copper oxide 2.5

 

i have fired this glaze a few times in a controlled cooling and gotten small purple crystals. i have a reminder to make a bucket of it and will not do that at all unless you have good news after your investigation.

 

the red glaze is a spectrum cone 4/6 lead free non toxic dinnerware safe glaze #1194, christmas red. the color of a santa claus suit. i have used it before and found its color great. recently the glaze had turned to a gel after i thinned it with water to use it in my sprayer. i contacted spectrum and bought some of their glaze thinner which is actually sodium methahexaseveralmoresyllableslong something or other. it is supposed to change the ph of the glaze. i used it and found that the color was only pinkish like the interior of a hothouse tomato, not the red i got when the bottle of glaze was new. whether that is due to my inexpert glaze spraying or the thinner, i do not know. all of the 8 pieces were a similar color. will appreciate any suggestions you have. thank you

[Guest JBaymore]

When we look at the glaze molecularly we find:

 

46.20 Custer Feldspar

14.20 Dolomite

18.00 Silica

5.00 Zinc Oxide

5.20 EP Kaolin

11.40 Gerstley Bor (99)

2.00 Bentonite

2.50 *copper oxide

2.50 *Cobalt oxide

 

0.35* CaO

0.23* MgO

0.15* K2O

0.09* Na2O

0.18* ZnO

0.00* P2O5

0.00 TiO2

0.32 Al2O3

0.13 B2O3

2.76 SiO2

0.00 Fe2O3

 

Cost: 0.18

Calculated LOI: 9.75

Imposed LOI:

Si:Al: 8.61

SiB:Al: 9.01

Thermal Expansion: 7.16

Formula Weight: 271.09

 

To greatly oversimplify here........

 

Silica (SiO2) is the main glass former for our glazes. Pure silica glass is very durable and you could say it is the "goal" against which we sort of measure the nature of a glaze. But its melting point is so darn high that we have to lower the melting point with fluxes. As we add fluxes, stuff like calcium oxide, sodium oxide, magnesium oxide, and so on, to lower the melting point the percentage of the overall glass that silica takes up gets less and less. As we decrease the silica content, the glass tends to become less durable and hard.

 

Ceramic chemistry has something called "limit formulas" for the various cone ranges. These limits say that "typical" and "good" glazes fired in a specific cone range usually have a range of the various molecular relationships of the oxides in a glaze in a certain range of values.

 

For cone 6 the typical range for the silica content can go up to about 4.7 mols. While the red highlighted silica number above is within the possible low end limit of 2.4 mols... it is JUST above that. That says that the glaze COULD hold a lot more silica in this cone range. So that silica number alone says tha tthe glass that is being formed is probably not all that durable. Not terrible by itself,.... but there are other factors to look at.

 

Alumina (Al2O3) has a number of functions in a glaze. An important one of them is to provide viscosity to the molten glass to keep it from flowing off the vertical sides of a form. But another aspect of alumina is it imparts hardness and chemical durablity to a glaze also. In particular, it helps to resist acids attacking the glaze. At the red highlighted 0.32 mols number above, it too is on the low end of the typical limit that can go as high as 0.64. Again it is above the lower limit of 0.29.... but just barely. So this number alone also portends less durability in the glass than might be possible. Not terrible, butr something that one might look at.

 

Then there is the factor that you have a portion of the overall glass forming function here coming from the boron glass former (B2O3). Like silica, boron is a glass former (resists re-crytalization after melting), but it is a much softer less durable glass than silica glass. SO this is potentially also contributing to a lack of durability in the overall glass.

 

When you put the three aspects together, AND when you add a high molecular equivalent of potentially toxic oxides into the melt (particularly the CoO), then there is room for concern about the ability of this glass to "hold" all of the oxides bound into the matrix. And one of those particualr colorant/fluxes is KNOWN to have some real issues with being held in a glaze melt and increasing the leaching of even stable glazes.... copper oxide.

 

When you then add in a test that SHOWS that there is a color change when exposed to acidic (or basic) materials .... well..... you get the answer. The glaze would have been "suspect" when looked at molecularly without ever firing it. The inadvertant test you did confirms this concern.

 

Additionally, both CoO and CuO are powerful fluxes in addtion to providing their respective coloring actions. As you can see in the above molecular analysis, I did not include them in the flux calculations. When they are taken into account appropriately as fluxes, the addition of yet MORE fluxing material then LOWERS the alumina and silica numbers even further. Quite possibly taking them out of the lower limits.

 

To potentially address this issue .... the task would be to bring the SiO2 and Al2O3 numbers up into at least the mid-range of the limits while maintaining the relationships of all the other oxides. But some of the "character" of this glaze likely depends on the current "mismatch" relationships. So fixing the durability might change the look of the glaze.

 

Hope all that helps explain it.

 

best,

 

.......................john

[Nancy S.]

Brandywine Science Center http://www.bsclab.co...lab_pottery.htm

 

 

Trying to access the page gives me a 404 error....

[Nancy S.]

So here's a "food safety" question I haven't found an answer to, and while the topic's on the table, here goes:

 

What about underglazes, mason stains, or oxides that do NOT have a clear coat? Let's say I want to make sugar keepers in different colors. Will they be food safe as long as there's no lead or cadmium in the colorant?? Brandywine's site (http://www.bsclab.com/Pottery_Testing.html) only mentions glazes.

[Guest JBaymore]

What about underglazes, mason stains, or oxides that do NOT have a clear coat? Let's say I want to make sugar keepers in different colors. Will they be food safe as long as there's no lead or cadmium in the colorant?? Brandywine's site (http://www.bsclab.com/Pottery_Testing.html) only mentions glazes.

 

It is possible for oxides to leach out of clay, glaze, slip, underglaze, and so on. The only way to know for sure is lab testing.

 

Remember that the homebrew "lemon slice" and "soak in vinegar" tests are merely "rule out" tests. A positive result there (color or surface change) simply tells you that you really do not need to go to the expense of doing lab testing; they will be bad. But a negative result from the homebrew test (no changes seen) does NOT tell you that the glaze is NOT leaching. Those home tests are not sensitive enough nor do they meet leagal standards for lead and cadmium bearing glazes.

 

I'd bet that if you put on a thick-ish layer of underglaze containing non-encapsulated cadmium compounds on a piece and then covered it with a thin coating of a "food safe" glaze... that the cadmium release would be there in a standard acetic acid leacing test after firing. And the higher you are firing the more likely this is to occur as the interface layer expands and the definition between glaze and body starts to become very "fuzzy".

 

Lead and cadmium compounds are the only oxides that are legally regulated by the US FDA at the moment. That does not say that there is no liability on harm done by any other potential toxicological issue. It is just the two that have regulations attached to them for food use in the USA.

 

best,

 

........................john

[oldlady]

john,

 

thank you for the clear explanation of the problems in the glaze recipe. i read it all and it makes sense. i am not a chemist and really cannot manage to think of one more complicated thing such as how to fix the recipe. i fire only 8-10 times a year so i do not have a way to "play" with the ingredients. (i would rather have a root canal)

 

if you have any suggestions i would welcome them. i know i have used copper carb at least once when i mixed this glaze since i had no copper oxide. and i have only had cobalt carb so no cobalt oxide it that batch, either. i recently got both oxides just so i could mix this glaze since i am down to what is left in the sprayer bottle and none in the bucket. wont be mixing it now.

 

have you any thoughts on the red glaze? it really is a pretty color and i would love to be able to use it....SAFELY!!

[pricklypotter]

john,

 

thank you for the clear explanation of the problems in the glaze recipe. i read it all and it makes sense. i am not a chemist and really cannot manage to think of one more complicated thing such as how to fix the recipe. i fire only 8-10 times a year so i do not have a way to "play" with the ingredients. (i would rather have a root canal)

 

if you have any suggestions i would welcome them. i know i have used copper carb at least once when i mixed this glaze since i had no copper oxide. and i have only had cobalt carb so no cobalt oxide it that batch, either. i recently got both oxides just so i could mix this glaze since i am down to what is left in the sprayer bottle and none in the bucket. wont be mixing it now.

 

have you any thoughts on the red glaze? it really is a pretty color and i would love to be able to use it....SAFELY!!

 

 

The green/blue glaze: using copper carb lowers the effective amount of copper in your glaze. if you use copper carb and want to keep the same 'recipe', you should use more since copper carb "=" copper ox + CO2 that burns off in the kiln (LOI).

 

i would go two ways:

 

1 does the glaze REALLY need so much copper and cobalt? the fact that you once used carb instead of ox and got the same colour response suggests that it doesn't. I would go in the direction of lowering the oxides and seeing if i still get the same colour response. cobalt too is a powerful colorant, 2.5% is a lot and you can probably do with less

 

after lowering the amount of oxides, try to make the glaze base itself more durable:

 

2 also, add clay (= alumina) and silica to see if i still get the same surface and colour response

 

each of those steps should ideally be done on a grid of 25-35 paper cup test batches, all test tiles fired in the same firing. in the end you will be able to choose a glaze that has the more or less same colour and surface but is safer to use and more durable too.

 

no advice about the red. there is a certain kind of red that i wouldn't use no matter how many times they put 'non toxic' on the label.