The following test is often used for the detection of alum in flour:--A small quantity of the suspected sample is made into a paste with a little water and mixed with a few drops of an alcoholic tincture of logwood; a little ammonium carbonate solution is then added. In the presence of alum, a lavender-blue coloured lake is formed, which often becomes more apparent upon allowing the mixture to remain at rest for a few hours. The production of a brown or pink coloration is an indication of the absence of alum. A modification of this test, proposed by Blyth, consists in immersing for several hours in the cold aqueous extract of the flour a strip of gelatine, with which the alum combines; the gelatine is subsequently submitted to the action of the logwood tincture and ammonium carbonate as above. For the quantitative estimation of alum in flour, the following processes are usually employed:--A considerable quantity of the sample is incinerated in a platinum dish, the ash is boiled with dilute hydrochloric acid and the solution filtered. The filtrate is next boiled and added to a concentrated solution of pure sodium hydroxide, the mixture being again boiled and afterwards filtered hot. A little sodium diphosphate is now added to the filtrate which is then slightly acidulated with hydrochloric acid, and finally made barely alkaline by addition of ammonium hydroxide. The resulting precipitate, which, in the presence of alum, consists of aluminium phosphate, is brought upon a filter, well washed, and then weighed.