Food Bearing Safety of Larger Bowls

[Asa Shows]

I have a question concerning the the food bearing safety of clear and colored blown glass vessels (tumblers and bowls in particular). I'll ramble on a bit about where I am at in my thinking on this and then get to the question.

From what I have been able to find so far (on the web and in this thread http://talk.craftweb.com/showthread.php?t=5937), only glasses containing lead or cadmium are of concern. All the other common ingredients including arsenic and antimony do not seem to be of concern in any regulated fashion because they are chemically bound and/or leach at rates far below the daily/weekly intake threshold for rats er humans. All the sources I was able to find (just searching on the web) agree that lead and cadmium will leach into water/beverages from the glass. The degree to which they leach and whether it is a dangerous level depends on the glass composition and the paper/standard you read.

I use Spectrum 96 nuggets for my clear which they have stated/tested as food safe. I also use a variety of color bars, frit, powder ... which unless stated otherwise on their site, certainly contain lead and depending on the color cadmium. To avoid any concern over leaching, I always take a first gather of clear Spectrum and then apply color. This has satisfied me for a long time, but recently I have been blowing larger bowls, (larger at least for me, around 18" in diameter and 9" deep) and am concerned about how thin the protective layer of clear is on the inside of the bowl. Because I like to calculate things, I went ahead calculated how thin the layer was.

My typical bowl involves a small first clear gather, followed directly be a color overlay, followed by clear/color... I estimate the volume of the first clear gather or rather the portion of that gather that gets into the final piece (since a decent portion of it stays stuck to the pipe) to be about a 1" diameter 1" high cylinder. To make it easy to calculate, I have assumed the final piece to be an 18" diameter 9" deep bowl, ie a half sphere.

Given this and assuming the clear spreads perfectly evenly we end up with a clear protective layer 0.039 mm thick or 3.9E-5 meters thick. This seems pretty small to me, but that is because I am pretty big compared to it. I am not enough of a chemist to look at it from the atomic level, best I could do, which is probably wildly inaccurate, is divide 3.9E-5 by 1.0E-10 (rough atomic sized number) and come up with 390,000. Meaning, ignoring lots and assuming more, there could be 390,000 atoms of clear safe glass between my fruit punch and the scary enamel white. Even then that doesn't really mean much to me.

I feel comfortable that the clear layer would protect food for some quantity of time. If I took one of these bowls down and had it tested for lead and cadmium assuming they were in the color overlay, I think the bowl would pass the test just as well as the Spectrum clear passed it. I am more concerned about whether it would continue to pass the test over time. Glass will dissolve in water/soda/juice over time but I have no idea at what rate. I tried to find some generalized number in some study somewhere for soda lime glasses but didn't.

Anyone have any thoughts, opinions or information/links/studies you could share with me on this? Is it a general practice in this community to make food safe vessels with a clear gather followed by an overlay? Do many of you even make food functional vessels or do you stay away from it (or at least doing it with lead colors) for this very reason? Is 3.9E-5 actually really thick from a molecular dissolution perspective?

Thanks in advance for any input.

[Ben Solwitz]

I've been told by various people that even high lead glasses aren't unsafe as long as you don't store things in them for long periods of time. For example you could drink wine (somewhat acidic) out of lead glass stemware, but if you stored wine for five years in a lead glass bottle, you might end up ingesting a lot of lead. I don't have any scientific references as to how quickly various compounds actually leach out of the glass, but that is what I've been told by various museum directors and curators and so forth.

Edit: Wikipedia has some relevant information and links to other sources on the Lead Crystal (http://en.wikipedia.org/wiki/Lead_crystal) page.

[Pete VanderLaan]

Tom Littleton is the man who really keeps up on this. The beverage container industry has a standard that does not allow either lead or cadmium decoration within a half inch of the lip of a tumbler or of stemware. Ben is correct that the only real significance revolves around long term storage of materials in leaded decanters.

That being said, your methodology should be just fine unless people start eating the chipped lip remains from your work should it be damaged while plowing through their crunchy granola. Glass is not porous. I would worry about how the bowls look and not the health hazards from using them,

[Tom Littleton]

It's actually closer to 3/4 of an inch.

However in California, a number of companies (McDonalds is one) have been sued under under a statute (I think it is proposition 65) for having lead based decoration on the outside of beverage containers (like the ones that are given away). This decoration is not close to the lip but is considered to expose the user to lead simply through skin contact. The only way to get around this is to label your work with a statement similar to "Work of art, not for use with food". Note that my wording is not the precise wording that is required by the law.

[Asa Shows]

I've been told by various people that even high lead glasses aren't unsafe as long as you don't store things in them for long periods of time. For example you could drink wine (somewhat acidic) out of lead glass stemware, but if you stored wine for five years in a lead glass bottle, you might end up ingesting a lot of lead. I don't have any scientific references as to how quickly various compounds actually leach out of the glass, but that is what I've been told by various museum directors and curators and so forth.

Edit: Wikipedia has some relevant information and links to other sources on the Lead Crystal (http://en.wikipedia.org/wiki/Lead_crystal) page.

Most of what I have read is hard to digest and withdraw a definitive yes or no without spending a lot of effort parsing the text and then actually testing the glass. Probably the best article I was able to find (with what seems like a reputable source to boot) is this one, http://www.coe.int/t/e/social_cohesion/soc-sp/public_health/food_contact/PS%20E%20LEAD%20LEACHING%20VERSION%201.pdf. It agrees with Ben and Pete in regards to lead glass (of certain compositions) and beverage contact being safe enough. All in all though, I think that the water is murky enough, for me, around this that I'd rather avoid the issue altogether. Hence the clear gather to start and one to finish (avoiding the 3/4" and proposition 65 issues). I just got to thinking that perhaps I wasn't avoiding the issue if the clear layer became too thin. Which led to the calculation and the question. It is good to hear that you don't think there should be a problem with the thickness of the clear layer Pete, as I respect your knowledge of glass chemistry.

That said, I'd still like to know if this is how others make food safe vessels. I also like numbers so if anyone has one hanging out regarding corrosion of the glass I'd still like to know. The closest # I was able to find was under Corrosion of Glasses on this page http://en.wikipedia.org/wiki/Corrosion. Stated,

"In typical natural conditions normalised corrosion rates of silicate glasses are very low and are of the order of 10-7 - 10-5 g/cm2 d. The very high durability of silicate glasses in water makes them suitable for hazardous and nuclear waste immobilisation."

Unfortunately I don't know what the mysterious d is and while the paragraphs preceding work at explaining it, I couldn't withdraw enough information to use it.

[Ben Solwitz]

It looks like d is time in days so taking the density of soda lime glass as 2.52g/cm3 from the wikipiedia page on glass (http://en.wikipedia.org/wiki/Glass#Physical_properties), we can calculate the amount of time it would take 3.9e-5 m of glass to corrode.

(10^-5 g/cm2 d) / (2.52 g/cm3) = 3.97e-6 cm/d = 3.97e-8 m/d

(3.9e-5 m) / (3.97e-8 m/d) = 982 days

Note that I used the conservative estimate of 10^-5 g/cm2 d so it could be as much as 98200 days. It sounds like that layer of clear will hold up for quite a while, but it's hard to say how long without really getting into the details of the glass composition, what environment it is stored in, etc.

[Asa Shows]

That simple eh? Nice. Rereading the section, you're right. I'm not sure why I didn't get that first time around, but you know it was late, the sun was in my eye and there was a gust of wind... No, I think I was trying to make it more complex by having the pH of the solution somehow factored into the d.

If we take your conservative number of 982 days and figure the bowl is used once a day for an hour (in the case of a serious pho family, uh that is a non-acidic, room temperature, natural conditions pho :), we end up with (982*24) / 365 = 64 years worth of daily use before the layer deteriorates. If we tweak a little and say they take a couple days off each week it looks more like 90 years or a decade or so shy of my intended lifetime. Brilliant, we have calculated away all my doubt .

Joking about the doubt, not sure if that remains or not. I'd agree, a definitive answer can't be had here by reading a small rather dense section on a publicly edited site and then running a number (with an unknown derivation/set of assumptions) to declare a that bowl avoids any possibility of lead leaching for 90+ years. But, it is interesting/fun.

[Scott Novota]

I would be willing to bet a large sum of money that if you had a glass bowl with 20 years of day to day use on it it would not last in one piece long enough to ever be much harm to anyone.

Glass tends to be drawn directly to hard surfaces as it get older.



Scott.
.

[Asa Shows]

Not something to depend on but yeah that is probably the way it would go.

[Steve Stadelman]

Asa, you have also left out the possibility of taking a second gather, stripping it a little if need be, and then starting the color application.

[David Patchen]

Steve, with that suggestion you've subjected all of us to another round of multiple involved calculations on the thickness of the clear in bizillionths of an inch and the lifetime of a cxxx-hair of clear glass! :)

Nevertheless I do like your suggestion--double the initial bubble forget save the calculations for the job at NASA. (Although they will say that it will take 9032 years to wear through your clear. It will be 2987% more likely someone will break the thing in the interrim as Scott pointed out.)

Blow away Asa!

[Garner Britt]

If I wanted to worry about it, I'd be concerned at the lip of the bowl or especially glasses.

Think about a simple gather-overlay-gather cup. after the transfer, one generally tweezes and/or trims. Even with no trimming, your overlay color is exposed. Depending on the color and it's "stiffness" it can be difficult to push that color back into the clear....bad description but I think you can follow...

garner

[Asa Shows]

Steve.

Yeah, making the first clear gather bigger obviously thickens up the final clear layer. There is a point however where it uses too much color and since I blow solo it becomes more difficult, at least for me, to do the overlay. In any case, I think would have run the calculation just because I didn't have an intuitive enough feel for how thin even a double dip/gather would end up. The 1" cylinder is on the conservative side of the gathers I do.

Garner.

While I don't trim my lips in general, I agree some color is exposed on the lip. Albiet a very thin strip, on the order of 3.9E-5 meters thick if you use a 1" cylinder of color:), provided like you say the color doesn't do something funny when opening. This doesn't concern me though because there is no food (particularly liquid food) sitting on the lip of the cup/bowl. The quantity of chemical leaching is proportionate to (among other things) the quantity of time the food is in contact and surface area of contact. Those both seem very, very small for the lip (and while I like to calculate, I don't think I'll run those #'s).

That said I don't know how that would set from a regulation standpoint as it appears from Pete/Tom's post that there can be no exposed leaded decoration within 3/4" of the lip of a tumbler. I would be curious to read the document/standard that states that if either of you has a link to it.

Another way to go about this, as I am sure many of you and certainly the container industry does, would be to just batch my own "food safe" colors and dispense with all of this all together. I am working on that. Along those lines, if I start with a durable lead/cadmium free base batch/cullet, it seems to follow that as long as I avoid adding lead and cadmium, I should end up with a food safe color. Is this a good assumption? Are there any other ingredients/colorants/opalizers that are in murky water (once they are in the glass that is, obviously I should be careful with all the colorants before they come out of the annealer)?

Asa

[Ben Solwitz]

Selenium is toxic, I'd give you more but I don't know much about batching. Glass Notes has a list of toxic ingredients but I'm not at home right now so I can't check, sorry.

[Asa Shows]

No problem, I've got Glass Notes along with board/web recommended books by Lynggaard, Simmingskold, and Weyl. Glass Notes mentions that all chemicals should be considered hazardous, the particularly notable ones being Barium, Lead, Zinc, Cadmium Sulphide, Selenium, Potassium Dichromate, Arsenic, and Antimony. The book "Raw materials for glass melting" by Bo Simmingskold gives a pretty detailed description of a large number of chemicals used to make glass and labels the toxic ones. They are talking about on the batching side of the process though. My question really pertains to the final glass product and whether any of these chemicals retain their toxicity. That is a very complex question from a chemical perspective. If I trust that Big Glass isn't as ruthless as the very ominous piano wire wielding Big Oil, then I could imagine the most common toxic ingredients have all been analyzed for leaching and have been found to be below tolerable levels. As such and provided I haven't missed any (insert organization here, ie FDA) set leaching from glass/ceramic limits on chemicals other than lead and cadmium, I guess I should color as I will.

[Larry Cazes]

If I trust that Big Glass isn't as ruthless as the very ominous piano wire wielding Big Oil, then I could imagine the most common toxic ingredients have all been analyzed for leaching and have been found to be below tolerable levels.

Asa, Which BIG GLASS are you referring to? Gaffer, Kugler, etc. ? Have you seen any endorsements from the color, batch, or cullet manufacturers endorsing their products as food safe? Why lose sleep over it. Just case everything intended to hold food in clear and be done with it :)

[Asa Shows]

Big Glass was referring more to the commercial container/dinnerware industry and was meant as a joke. I don't think there is any conspiracy here. I am not losing any sleep over this and I am glad to here you think casing is a good way to go. An evaluation of that is what this thread started as. I just broadened it with a related (although not mutually exclusive) alternative. The more you know, you know.

[Pete VanderLaan]

I do think you are being overly concerned here. Glassnotes does say that all of this stuff is toxic but I think that too is an overreaction on Henry's part and I did write most of the sections on glass batching for glassnotes. Henry is simply covering his ass.

You do need to stay away from lead. Not so much as a food concern but for your personal health. Lead monosilicate is the only form I will work with since you can actually get it on you without immediate risk. The oxides and carbonates are to be avoided completely.

Cadmium should be avoided unless you know what you are doing -period. Selenium, while not nearly as toxic should be avoided for the same reasons. Fortunately, Selenium really smells bad when it is not vented properly.
Barium is more of a handling issue. It is in Spruce Pine after all as well as GLASMA batch glasses and many cullets.

Having books is really good, but it is not enough. You really do need a guide unless you like rediscovering the wheel interspersed with occasional trips to the ER. Since the wheel can be quite toxic, the guide is a good plan.

You also need good ventilation. By good, I mean an air change every two minutes and no less. My own studio can ventilate an air change every twenty seconds and I use that capacity in toxic melts.

But you can do some straighforward melts. Copper reds, silver ambers, cobalt and copper blues and some violets are quite accessible without a ton of knowledge. Forget bright reds and yellows. Greens are carcinogenic if derived from Chrome and Chrome is seriously underrated as a toxin.

But really, the food issues seems to me to be a red herring. In all the years of making glass I have behind me, I have yet to be made aware of a small studio getting assaulted for this issue. Maybe it's out there but I don't know about it. What is really hard is making glass that is attractive enough to sell in the first place.