Our six-month-old daughter loves to chew on metal spoons. She already has two teeth. Is this harmful to her teeth and dentition? Is a hard plastic spoon less harmful despite small plastic particles that can come off?
What is with the ability of harder material to scratch softer material Mohs scale. Teeth are out of Calcite (most of the teeth). Calcite has a 1.75 Mohs hardness. Steel has a Mohs hardness of 4–4.5. So for me this means if steel is chew with Calcite the Spoon must damage the teeth. But maybe this does not affect very smooth objects like spoons?
First off, the best person to ask about this for your baby is your pediatrician, as they can make sure that she doesn't have anything out of the ordinary going on in terms of dentition.
In general, teething on food-safe metal objects that are meant for the mouth is totally fine, as far as I've been able to find. Spoons, cups, etc., all went into my children's mouths on a regular basis and were well chewed. They're great for teething, especially when they start out cold - you can put it in the fridge for a minute, or in cold water, to make it even better as it reduces swelling.
I looked around, and found that dentists generally seem to consider metal spoons a good solution to teething. This article by Periodontist Todd Welch that specifically recommends cold metal spoons for teething symptoms. The same (word for word) recommendation appears on OralAnswers, which is also run by a dentist ("Tom"). Not sure which site posted it first. MouthHealthy.org, from the American Dental Association, has a similar recommendation as well.
One warning I read in several places, like this article, was that metal spoons are potentially dangerous if the child is walking around with it, because they could fall and injure their mouth. A six month old probably isn't walking yet, but they may be cruising, so you may want to keep it to high chair or other highly supervised times. I've never heard of this, nor have I seen any concrete evidence it's a risk, but it pops up in several articles.
If you're concerned about that, there are some teething-safe metal options. Don't let her chew on your keys, because they could contain lead and are likely dirty, but there are some purpose-designed metal teething object. Keynimals for example we used briefly, as an example, but any brand will do so long as it's specifically designed for babies to teeth on. Don't let her teeth on any metal object not designed to be used orally, as it may contain lead or other unsafe materials.
Finally, if you do worry about metal, my recommendation is silicone, not plastic. Silicone will not break off pieces, as plastic might as you note, as long as you pay attention to make sure nothing large breaks off. You can find freezable silicone teethers that may satisfy her need for cold contact on her gums as well or better than metal. A frozen washcloth may also feel good (wet a washcloth, put it in the freezer, then let her chew on it once it's nice and crackly).
Your daughter could chip her teeth on the spoon. I would recommend you try to substitute an alternative plastic spoon whenever she does this.
She might be simply enjoying the noise it makes when she bites on the metal, but I think it should be discouraged if at all possible.
"Food should be given from a hard plastic weaning spoon that will not crack. Do not use a metal spoon"
"A metal spoon may harm the baby’s mouth."