To make a Sack posset, according to The Cooks Guide: Or, Rare Receipts for Cookery, 1654, 'Take a quart of thick cream, boyle it with whole spice, then take sixteen eggs, yolks and whites beaten very well, then heat about three quarters of a pint of sack, and mingle well with your eggs, then stir them into your cream, and sweeten it, then cover it up close for half an hour or more over a seething pot of water or over very slow embers, in a bason, and it will become like a cheese.'
It is uncommon for posset pots of this type to retain their original covers. See Louis L. Lipski and Michael Archer, Dated English Delftware, Tin-Glazed Earthenware 1600-1800, London, 1984, pp. 203-205, for posset pots of this form and other examples with this type of painting. A posset pot decorated in this manner, though of a bulbous form, was in the Harriet Carlton Goldweitz Collection, sold in these rooms, January 20, 2006, lot 28, and a further example was sold Sotheby's London, November 21 2006, lot 37.