Kamiya Pocket Warmer Ash Type Hand Warmer

Kamiya Pocket Warmer (Kamiya Kairo) is a traditional ash-type pocket warmer.

I bought it on Yahoo Auctions a few years ago but never used it.
The inside is very clean.
The outside is difficult to open. I used a flathead screwdriver in the gap on the side to open it. (I'm not sure if it's because I don't have enough strength or if it's genuinely difficult to open.) The double-layered lock inside engages properly.

I heard that the charcoal used for incense ceremonies could be used, so I bought some with the intention of using it.