This section is from the book "Hygiene Of The Nursery", by Louis Starr. Also available from Amazon: Hygiene of the nursery.
These plasters are used for the purpose of making counterirritation, and must be graduated in strength according to the tenderness of the skin and the end to be accomplished. Pure mustard is very irritating and will quickly blister the tender skin of a child. Flour is the ordinary diluent, and the strength of the plaster usually ranges from one part of mustard to three, six, or even more parts of wheat flour.
In making the plaster, take one teaspoonful of mustard flour and add to it three teaspoonfuls of wheat flour; mix them together thoroughly on a plate, and put on as much hot water (never vinegar) as may be necessary to make a soft mass. Spread evenly over a piece of muslin. To prevent the mustard from adhering to the skin, place a piece of gauze or thin muslin over the surface of the plaster; turn down the edges as in poultices.
Remove the plaster after the surface becomes quite red, usually three to five minutes.
 
Continue to: