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These are excellent examples of some of the earliest coasters for decanters, dating from the mid-18th century. Quite typically they have wavy edges and are quite shallow. They are made of mahogany and unusually these are not split along the grain of the wood as so many are. However, their best attribute is the fine line of brass stringing running around the top edge.
Coasters are an important wine accessory. Not only do they prevent drips from a decanter from soiling a tablecloth, they also can prevent decanters from cracking against each other.
Originally they would have been put on the table after the dinner had been served and cleared (voided was the word they used at the time). The ladies would have withdrawn (to the withdrawing room, of course) and the gentlemen would circulate wines or port around the table (now without its tablecloth). To run smoothly across a polished table, coasters were fitted with baize to their undersides, to ease their passage between one man and his neighbour. Unsurprisingly they were also known as decanter slides.
The coasters are each large enough to take a magnum decanter with a maximum diameter of 7.1", 18.1 cm, and with internal diameters of 5.5", 16.5 cm. We have several magnums and all of them fit easily.
There is a small difference in depth of colour, but it is much less noticeable on the table than it is in our images.