It does, eventually. For this type of project don’t use a microwave that you plan on ever using for food - or anything else - ever again.

I don't know what you mean by 100% efficiency.

The magnetron burns out from a high standing wave ratio, which is the ratio of how much energy bounces back into it, to how much its outputting. If you aren't heating anything up, the microwaves bounce back and cause damage.

If you have something in the oven that absorbs the microwaves and gets hot, then you're fine. Liquid water does this, including the water in your food, but ceramics do it too. That is why when you cook frozen foods on a ceramic plate, the plate can heat up faster than your food. Once the food defrosts, it'll have liquid water and begin cooking at full speed. Sometimes the food only defrosts from the plate heating up, so you end up with food cooking from the bottom up, instead of the top down.

This is what I meant: does the ceramic crucible absorb energy as efficiently as water… or is the magnetron considered disposable in this case?

According to the other comment, it is indeed disposable.

I don't think eugenekay's reply is helpful, unless in the pedantic sense:

yes every microwave oven eventually fails.

yes it is good practice to dedicate a microwave for food and a microwave for science experiments.

There's enough ceramic to absorb enough microwaves. There's other reasons the heat could ruin the microwave, and plenty of reasons not to share food with a microwave that has vapors from melted colored glass. Considering how cheap they are second-hand, I wouldn't use a kitchen microwave for it, especially so if there's colorants in the glass.

As long as there is a good resistive microwave absorber in or on the inside of the kiln, and as long as the kiln insulating properties suffice, why would the magnetron burn out?