What finish should I use to make a bowl “microwave-safe”?

I want to give a turned bowl to someone as a gift, but I know they will inevitably use these bowls in the microwave.

From my own experience of force-drying wood in the microwave, I know it can get hot to the touch after about 30 seconds, depending on moisture level.

Are there any finish solutions for making "microwave-safe" wood items for serving food on?

• If microwave is a concern, then dishwasher should be too... – guitarthrower Apr 11 '16 at 16:35

Are there any finish solutions for making "microwave-safe" wood items for serving food on?

It's not so much a question of "microwave-safe" finishes as "microwave-safe" materials.

Microwaves work by exciting water molecules, thereby increasing their energy (i.e., heating them up). The reason your wooden bowl gets hot is because of the water trapped in the wood fibers.

I would strongly advise against microwaving a finished wood bowl for not only the risk of burning one's self on it, but also because the rapid heating of the water in the bowl can cause it to crack.

I want to give a turned bowl to someone as a gift, but I know they will inevitably use these bowls in the microwave.

Very clearly tell the intended recipient that wooden bowls are not microwave safe. If someone uses the bowl improperly and it breaks, yes, that sucks for them, but you're not at fault having told them. Common sense has to factor into things sometimes.

By the way, not only would I tell them to not microwave the bowl, but don't soak it in water, and don't put it through the dishwasher either.

• If the wood in the bowl is dry it won't heat up and it won't crack (any faster than normal or defects). I know it seems like common sense not to microwave them, but it's really no different then putting hot soup in a wooden bowl. Keep in mind that people used wooden bowls to eat hot food for thousands of years. While microwaves are a new technology. If there is food in the bowl the food will heat up way faster than the wooden bowl. Dishwasher on the other hand is totally different. That's more like cooking a wooden bowl because it's an extreme environment. – Reactgular Mar 15 '16 at 17:53
• If the wood in the bowl is dry it won't heat up and it won't crack Even "dry" wood has moisture in it. It's near impossible to remove all moisture from a piece of wood. Eating hot food isn't really an issue because you're not turning the entrained water in the bowl to steam like you would with a microwave. – grfrazee Mar 15 '16 at 17:55
• yes, I understand but it's all relative. Put wood in the microwave for 2 minutes with nothing else and it will heat up. Put wood with food on it for 2 minutes and the food will heat but the wood won't. It's part of the way microwaves work. It's not that the wood will not heat up. It's just at a rate relative to the other material that has more moisture. It's something you can test yourself. Don't have to take my word for it. – Reactgular Mar 15 '16 at 17:59
• @ThinkingMedia Good to see that you are considering how a wooden bowl is going to react to microwaving when full of moist food from how it would react when empty. Most of the answers seem to be based on what happens to empty wooden vessels when being force-dried in a microwave oven. – Ast Pace Mar 15 '16 at 21:07
• @ThinkingMedia, "Put wood with food on it for 2 minutes and the food will heat but the wood won't." You're just theorising. It would be easy to test that for yourself but I guarantee you'll find the bowl will heat up, it's not much different to heating food in certain ceramics which are not rated for microwave use, where both the food and the container heat up. – Graphus Mar 16 '16 at 15:27

I would seriously suggest you just get them a glass or plastic bowl if you want to gift them one you know they'll want to use in a microwave :-)

Regular microwaving is not a good environment for wood since it'll heat it and drive out moisture. Up to a point this doesn't do any harm as you'll have some idea about from your force-drying experience, but if you've ever overdone the microwaving as I have (causing the water in the wood to boil) you'll know there are some issues.

First thing I'll mention is the one that will be most immediately apparent: the smell. If they do overheat the bowl even just once and it boils the intrinsic water they'll create wood steam and the smell of this is not exactly conducive to fine dining (think paper mill and you'll have some idea)! Worst still, that odour will linger in the microwave for months. Trust me that's no exaggeration, I speak from experience.

In terms of the safety of the bowl, past a certain level and the wood will end up way dryer than is natural. With repeat use the moisture level will cycle up and down excessively (most especially if they wash it). I think you could bet money on this leading to warping or cracking, even if the wood used in the turning was straight-grained and clear.

More seriously given enough microwave exposure there's even a risk of fire. If the wood happened to be very dry already, from being microwaved recently and/or because of local conditions, and it was then microwaved for a good amount of time you could get localised scorching. And once the wood is charred it's just a small step to ignition (same basic principle as char cloth).

I have stained my cutting boards and place hot pizza pans on them with no effect. However they still will warp with constant water left on them.

I would suggest you have two options. Rubbing mineral spirits or butchers block oil and finish with cheesecloth or tack cloth.

When in doubt test on the wood in question. Stain a scrap of wood then buy a cheap heat gun. Use the heat gun for a prolonged amount of time to see its affects. This will prevent any damages to actual equipment tests. It will also assure you of your friends sanity.

On food contacted stains I like it to rest for a month if not weeks to make sure the stain completely soaks in.

Also make sure to take a plain cloth and wipe any access stain buildup. You usually do this after 24 hours of curing..

• Heat soak from a hot pan sitting on a wooden surface is much different than the internal heating created by a microwave. A hot pan may scorch the surface, and will cause heating of the wood, but the microwave will heat the moisture in the wood more-or-less evenly through out, and much more quickly. It will likely cause the internal water to boil. – FreeMan Mar 17 '16 at 13:08

While you can often get away with wood in a microwave you shouldn't. Or if you do, mass-produced cheap things but not something presumably valued. Similarly for the dishwasher - I put some wooden things in there but nothing that couldn't be replaced cheaply and easily.

But to be honest, finished wood and hot wet foods don't mix well, so using it in the microwave would be rather pointless (except I suppose for warming bread).

Varnishes tend to crack or peel while oils (even food safe ones) can give a taste to the food.

When creating wood products that will be used with food. Either dry, wet or microwaved the guidelines are generally the same for all the above.

The microwave doesn't change anything. The wood item will simply wear out quicker if it is microwaved. The point here is to keep it non-toxic and food safe. Microwaved, dishwasher or washed in the sink. You don't want to leach any poisons into the food as a result.

I make butcher blocks. They are easy to make and people love them as gifts. Here's what I follow as guidelines.

• Select a hard wood to ensure durability and reduce the pick up of moisture that can lead to growth of mold.
• Use non-toxic wood, and yes there are hardwoods that contain poisons. I recommend cherry, maple, oak and chestnut.
• Use food grade wood glue. You can buy non-toxic wood glue at most hardware stores now.
• The only natural non-toxic finish I know of is bees wax. It seals and provides a semi-gloss finish.
• While bees wax is the most recommended. I've not liked the results. What I use is butcher block finishing oil. It's non-toxic and food safe.

Some of my blocks are over 5 years old and still safe to use.

You can make a wood bowl microwave safe: put it in a Faraday cage. This means you coat the entire surface with metal, with holes less than the max of 1.2 cm or the distance between the holes and the wood: consumer microwaves use 12 cm wavelength, and apparently gaps of 10% of the wavelength are pretty opaque to it.

Such a continuous conductor will shield the wood against the microwave radio waves.

It won't be much of a wood bowl anymore, however.

At that point, your problem is reduced to protecting the bowl from the heat of the food. Also, the bowl will now block microwaves, which means heating will only occur from the "top down".

• ...never mind that you're now putting METAL in a microwave? Can't tell if trolling or... – Doresoom Mar 16 '16 at 20:23