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This is a very rare coaster because it will take a double magnum decanter (see illustration). It has a bulbous elevation and a gadrooned rim, while the base is turned wood with a central silver 'button' upon which a crest can be engraved.
Coasters made of Old Sheffield Plate must have a seam where the sheet of copper 'sandwiched' between silver meets and is joined. The important thing is that the silver which is visible is sterling alloy (92.5% silver) which is much more resistant to wear than electro-deposited silver. It is also an indicator of its age as electroplating superseded the OSP in the 1840s on account of its being much cheaper to produce. It made the OSP process obsolete at a stroke.
While there is a little copper showing in this coaster the large percentage of the metal showing is silver; it is in good condition.
Dimensions: 7.6", 19.3 cm. overall diameter, 6.4", 16.2 cm internally