The CWI Cookbook

 1 oz. dry mustard
3 tbs. milk or cream
1/2 tsp. salt
1/2 tsp. sugar

Mix very gradually, and rub together in a mortar, an ounce of flour of mustard, with three table-spoonfuls of milk (cream is better), half a tea-spoonful of salt, and the same of sugar; rub them well together until quite smooth.
Obs. Mustard made in this manner is not at all bitter, and is therefore instantly ready for the table.
N.B. It has been said that flour of mustard is sometimes adulterated with common flour, &c. &c.

From The Cook’s Oracle by William Kitchiner, MD, New York, 1829

Comment: “Flour of mustard” is simply finely-ground mustard seed, sold today as “dry mustard.” As might be expected of the days before the development of mechanical grinding mills for such things, the process had to be done by hand with a mortar and pestle, explaining the motives for dishonest vendors to step on the resulting powder with cheaper stuff. Pure food laws lay far in the future as Dr. Kitchiner wrote.

“Made mustard” as we know it today was just starting to come on the market in the 19th century. Makers competed as much to find distinctive bottles to make their wares stand out on store shelves, as they did over the recipe for the contents. A thriving subculture of mustard-bottle collectors similarly competes today to preserve these unique items from the ravages of time.