We were sitting in the other room watching television when there was a massive explosion in the kitchen. We rushed in expecting to find shards of glass and plums all over everything. Instead we saw the pyrex baking dish sitting quietly on the wire rack of the open oven. Apparently the alchohol fumes seeped out of the dish as it heated up and built up in the oven with enough force to blow open the oven door but didn't affect the baking dish. Word of advice that this method of making plum jam is not recommended.
Instead, I make "Kanro-ni" 甘露煮. I just make a simple syrup (equal amounts of sugar and water heated to dissolve the sugar) and just simmer the plums on a very low flame for 30 minutes or until the plum is soft. This is rather sweet and can be used as a sort-of snack when you are having Japanese green tea or you can serve this in-between dishes as a palate cleanser as shown above.
I store this in a glass container with a tight lid in a refrigerator. As long as you use a simple syrup (very sweet), this will keep a long time. Actually, this one is two years old. Other usages of the Umeshu plums is to make it into jam. I see that you can use the Umeshu plums in stewing meat such as pork or chikcen, which I have not tried.