This section is from the book "Materia Medica And Therapeutics Inorganic Substances", by Charles D. F. Phillips. Also available from Amazon: Materia medica and therapeutics.
A weak lotion of bicarbonate of potash (or of soda), 30 to 60 gr. in the pint, will often relieve the early discharging stages of eczema, and a stronger application (caustic potash, 5 to 20 gr. in the ounce) is a useful stimulant to patches in the chronic stage; although painful, it markedly relieves the itching, which is often worse than pain. The German school especially have reduced to a system the application of potash, in the form of their sapo viridis ("schmeier-seife"), which is made by boiling some animal oil with potash and its carbonate; it forms a soft amber-green compound, more elegant than our "soft soap." Of this a general bath is prepared with 1 dr. to the pint, a second strength (1 dr. to 1/2 oz. of water) is used for infiltrated subacute patches, and a third (1 dr. to 2 dr. of water) acts as a caustic for very chronic cases; besides these the German codex contains a "spirit of soap," etc. The solution of selected strength should be thoroughly brushed in, and the irritation quickly relieved by a stream of cold water. The use of such remedies is painful, and causes profuse serous secretion from the part; before commencing a course of them, vascular irritation should be subdued by cold water, etc., and afterward it will be found desirable to use some emollient, such as glycerin or oil, otherwise the skin becomes harsh and dry. There can be no doubt that in some chronic forms, and especially in chronic eczema of the scalp, the soft-soap treatment gives remarkably good results (Medical Times, i., 1860.)