I've had several French butter dishes over the years but I never knew how, exactly, to use them.  So I would buy one because it was pretty and after awhile donate it somewhere and then come across another one, buy it, admire it …. and so on.  

In the summer I came across one at a garage sale and googled sufficiently to understand how to use it.

Well, I quite liked the fact of soft butter kept fresh on the table without need of refrigeration.

It wasn't a pretty dish and the memory of the ones I had had that were of a nicer shape and colour got me watching for them on my usual route of garage sales and church thrift shops.  

This one appeared at the same time that the plain white with a blue stripe one kept having the butter slip out of the cup part and into the water.

Upon close investigation I realized that this attractive butter dish had a slight recess in the cup part (on the left) which gave a bit of a foothold, so to speak, for the butter.  I did not come across mention of this feature on any of the google sites but its presence makes me think the problem was not just that I had slippery butter.  !

What you do is pack butter into the cup part and fill the part on the right with water (the line is likely meant for that level: the cup lip is to be in the water to provide an oxygen-tight seal; also for unsalted butter you add a 1/2 tsp or so of salt to the water although I don't really see the sense of this).

At Friday's church sale, in with a box of tools, I found the item on the left which looked exactly like the top of a French butter dish – recess included!

Much searching around the tools etc. did not uncover what looked like a match to the top. A glass flower pot, glasses, pottery bottoms etc.were considered. 

This vase – I think it is meant for violets – got chosen.

I am pleased.

The water is clear;  the camera or the light or both have given crystal lines.