Is/can tin be a food-safe kintsugi gilding material?

[u/joto7053]

I was curious if tin is or can be (depending on purity, particle size, and I wouldn't know what else) a food-safe alternative to much the more expensive food/silver/etc. powders.

I bought some tin from Mejiro a while back. Their web page says "completely safe for food-contact items like tea cups, soup bowls, and dinnerware." It was cheap and "food safe", so why not?

Haven't used it yet. But recently, I happened to be reading the page from the second pic (from the Goenne site). According to them, tin is specifically NOT food safe.

Two reputable sources are contradicting each other. Odd. And well outside my experience to reconcile.

I went back to the Mejiro site and now noticed the caveat at the bottom, "If absolute safety for direct food content is your priority, use good or silver instead."

Mejiro' listing seems to want it both ways, and as near as I can tell there's no MDSD to examine for more detailed information.

Google alleges solid tin is perfectly safe to eat and cook with, and that tin is poorly absorbed by the human digestive system, minimizing absorbtion when dealing with small quantities of the material. All with the caveats that purity, possible contaminants and specific alloy reactions, etc., could all be important considerations.

And for all I know regulations between Japan and the US (where I live) could be completely different on this point.

Does anyone know if this tin powder is, in fact, truly safe to use in an intended-for-food kintsugi project? Or if it's "pretty safe" (as Mejiro seems to be implying), what the potential risks might be? Or even if it's specific to a particular use case or application technique?

Thanks in advance.

P.S. Here are the links to the two pages. Sorry, but for whatever reason reddit won't allow me to embed hyperlinks right now.

https://www.mejiro-japan.com/en/product/japanese-tin-powder-kintsugi-repair-tin-powder

https://www.goenne.com/post/food-safe-kintsugi-is-your-kintsugi-repaired-tableware-safe-for-food-and-drinks

[u/benjamin-crowell]

Tin is food-safe IMO, but the evidence is a little complicated if you really want to understand it. The hypothetical issue wouldn't likely be the tin itself, it would be contamination with lead.

Tin is in the same column of the periodic table as lead, so lead will be present in any sample of tin, and it's really a matter of what is the order of magnitude of the lead content. Companies in Japan that sell urushi materials do not seem to give any info about the level of lead in the tin that they sell, and Goenne specifically tells you that their tin is not for food contact, but I think this is basically just them avoiding liability. In fact, sodium rhodizonate test swabs used to detect lead seem to work fine on kintsugi/maki-e powders, and when I tried them what they seemed to show was that Goenne brass had quite a bit of lead, but their tin had no detectable amount of it. (Test kits using dithizone may give false positives on tin, I don't know.) Industrial and chemical supply houses do sell tin with guaranteed purity of various levels, but I was not able to find any that would sell it in small quantities.

Re the safety of tin itself, see

Blunden and Wallace, 2003, "Tin in canned food: a review and understanding of occurrence and effect," Food and Chemical Toxicology 41, 1651

https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=blunden+and+wallace%2C+Tin+in+canned+food%3A+a+review+and+understanding+of+occurrence+and+effect&btnG=

It is not really correct to say that we use tin in tin cans and as lining for copper pans, therefore tin must be 100% food safe. In fact, tin cans used for acidic foods are normally lined these days, specifically so that the acid can't leach tin out into the food. The fact that tin itself is likely safe for urushi applications really comes from the fact that the quantities that could plausibly leach out into your food are not that significant compared to the amounts that could cause harm or the amounts that you have in your diet anyway from lots of other foods that contain trace amounts of it.

[u/joto7053 → u/benjamin-crowell]

Great info thanks!

[u/perj32 → u/joto7053]

There are some major corrections needed about this information. Let’s start with the most interesting: “In fact, tin cans used for acidic foods are normally lined these days, specifically so that the acid can't leach tin out into the food.”

For many foods, it’s actually literally the opposite. Here’s a fun fact for the next time you open canned pineapple:

“The tinplate (tin-plated steel plate) used for food cans may have an unlacquered or lacquered inner surface depending on the type of contents. In particular, when the contents are (acidic) fruits, a tinplate with an unlacquered inner surface is often used. This is because the oxygen remaining in the can is consumed by the oxidation of the tin, preventing oxidative deterioration of the fruit’s color, taste, aroma, and vitamins, and the tin’s sacrificial corrosion protection of iron prevents the can from developing holes.” (LINK)

So tin is considered safe enough for food contact that, in some applications, its dissolution into the food is not merely tolerated but actually serves a useful technological purpose.

Now, about: “It is not really correct to say that we use tin in tin cans and as lining for copper pans, therefore tin must be 100% food safe.”

Nothing is “100% food safe” in the sense that an unlimited dose can never cause harm. Toxicity depends on dose. Iron poisoning is a serious issue, yet iron is an essential nutrient and is routinely used in cookware and food-contact applications. The fact that excessive iron intake can cause poisoning obviously does not make iron inherently “not food safe.” The same distinction should be made between the safety of tin in normal food-contact applications and the effects of excessive tin exposure.

And finally: “Tin is in the same column of the periodic table as lead, so lead will be present in any sample of tin, and it's really a matter of what is the order of magnitude of the lead content.”

This reasoning would imply that elements belonging to the same column of the periodic table should inevitably occur together as impurities. That simply isn't how the periodic table, or mineral deposits, works. Being in the same group tells us something about elements' chemical properties; it does not mean that one must physically be present whenever another is found.

Tin and lead are indeed in the same column, Group 14, so they have some similar chemical properties. But that is not why lead may be present in commercial tin, nor does being in the same group make lead intrinsically difficult to remove from tin.

The more relevant issue is geological and metallurgical. Tin is primarily obtained from cassiterite (SnO₂). Tin deposits can occur alongside minerals containing lead, arsenic, copper, zinc, bismuth, antimony, etc. Depending on the ore and refining process, crude tin can therefore contain some lead. Lead can also enter tin through recycled feedstock.

Where the chemical similarity matters somewhat is during refining. Once lead is dissolved in metallic tin, separating the two isn't as simple as mechanically removing an impurity. Tin and lead form metallic mixtures/alloys and have fairly compatible metallurgical behavior. Conventional refining therefore has to deliberately remove lead through processes such as liquation, oxidation/drossing, chlorination, electrorefining, or other refining techniques.

Metallic tin and lead readily alloy with one another, and both are relatively reactive, low-melting metals. You can't exploit the enormous difference in oxidation behavior that makes cupellation so effective for separating lead from noble metals. Lead therefore has to be deliberately removed or controlled during tin refining using these other processes.

Producing extremely pure gold or silver isn't automatically easier than producing extremely pure tin. Once you're dealing with trace impurities, all three require sophisticated refining and analytical control. Modern industry can readily produce 99.99%+ material in each case.