This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Opium plaster is made by mixing powdered opium, or still better the extract in half the quantity, with boiling water, incorporating the mixture with melted Burgundy pitch and lead plaster, and then evaporating the moisture. It may be used, spread on leather, as an anodyne application in fixed pains, of a rheumatic character or otherwise.