Why Sauna Buckets Have Plastic Liners

It is not about safety

The usual assumption is that the plastic is there to stop the wood rotting, or because wood cannot hold water. Neither is really it. Wooden buckets held water for several thousand years before polypropylene existed.

A plastic liner is there because it is cheaper than making a bucket that does not need one.

What a liner lets a maker skip

A coopered vessel that holds water on its own has to be built properly. The staves have to be jointed accurately enough to seal against each other. The wood has to be dried to the right moisture content, because wood that is too wet when it is assembled will shrink and open up. The base has to seat into the groove tightly. The hoops have to be tight and able to stay tight.

Put a plastic cup inside and none of that has to be true. The joints can be loose. The wood can be whatever was cheapest. The bucket becomes a decorative sleeve around the thing that actually does the work.

That is the real tell. A plastic liner is not a feature, it is permission to build the wooden part badly.

So why does ours have a steel one?

Because we are not going to pretend otherwise: our bucket has a liner too. It is 304 stainless steel, deep drawn in one piece, and it lifts out.

The honest reason is that a lined bucket is more forgiving for a customer who stores it badly, and an unlined coopered bucket that is allowed to dry out completely will leak until it swells again. That is a real trade and we made it, because we too wanted convenience rather than something else to meticulously maintain.

What we are not willing to do is use the liner as cover. The staves underneath are jointed properly, the base is dowelled with no adhesive, and the wood is specified and measured rather than assumed. You can lift the liner out and look.

The part that actually bothers us about plastic

It is not that polypropylene is dangerous. It might be. We don't know for sure.

It is that the entire room is wood and stone and water, and then there is one plastic object in it. And that nobody making those inserts has tested them for twenty years of 190°F and steam, because they were not designed for that. Nobody has answered the question, which is different from the answer being fine.