Robert Newlands, of Wellington, asks :-

What are the constituent chemicals in urine? The ancient Romans used it as part of a cloth dyeing process.

Gerald Smith, a chemist at Victoria University of Wellington, responded.

Urine produces ammonia from the urea it contains and as such it is a source of water-soluble nitrogen essential for all living organisms.

The chemical basis for the use of urine in dyeing textiles is often poorly defined. The ammonia may, in some instances, remove grease and other cases, remove water-resistant coatings on the textile fibres which prevent the water soluble dye entering the fibres. Ammonia may also help in the extraction of the dye from the raw material which was usually parts of certain plants.

Where the chemistry for using urine in textile dyeing is well established is in the production of alum mordant for fixing natural dyes to wool so they don’t wash off. A very large industry producing alum from shale was set up in England in the 18th and 19th centuries. Shale, a rock found on the coast of Yorkshire, contains aluminium and when the rock is heated and soaked in water, aluminium sulphate can be obtained by crystallization. Then the aluminium sulphate was treated with urine to produce ammonium (from urea) aluminium sulphate, commonly known “alum”. The aluminium ions in this material form chemical bonds between both the dye and the fibre which strongly fixes or binds the dye to the fibre.

Alum was of huge strategic importance because it was essential for dyeing before the advent of synthetic chemical dyes. In some parts of the world naturally-occurring deposits are found and such was the demand for it, wars were fought to profit from its trade. Elsewhere it needed to be produced in the way described from urine and shale rock.