We've all heard about them and perhaps some of you have used one but I think for most of us today this is definitely a thing of the past. Below you'll find the recipe for a Mustard Plaster from "New Receipts for Cooking" by Miss Leslie ©1854.
MUSTARD PLASTERS.—Mustard plasters are frequently very efficacious in rheumatic or other pains occasioned by cold. It is best to make them entirely of mustard and vinegar without any mixture of flour. They should be spread between two pieces of thin muslin, and bound on the part affected. As soon as the irritation or burning becomes uncomfortable, take off the plaster. They should never remain on longer than twenty minutes ; as by that time the beneficial effect will be produced, if at all. When a mustard plaster has been taken off, wash the part tenderly with a sponge and warm water. If the irritation on the skin continues troublesome, apply successive poultices of grated bread-crumbs wetted with lead water.
A mustard plaster behind the ear will often remove a toothache, earache, or a rheumatic pain in the head. Applied to the wrists they will frequently check an ague-fit, if put on as soon as the first symptoms of chill evince themselves.
Definition of ague-fit
An obsolete term for a chill following a fever, which is said to be typical of malaria. This term is not used to working medical parlance, though it continues to be used by laypersons.
Source: Medical Free Dictionary, http://medical-dictionary.thefreedictionary.com/ague+fit