To confirm these the following chemical tests may be applied. Boil the ash left in the crucible with a little strong hydrochloric acid and dilute with water. Pass a current of sulphuretted hydrogen gas through the solution, if there be any tin present a brown precipitate of tin sulphide will be obtained. This can be filtered off. The filtrate is boiled for a short time with nitric acid, and ammonia is added to the solution when alumina is thrown down as a white, gelatinous precipitate, iron is thrown down as a brown red, bulky precipitate, while chrome is thrown down as a greyish-looking, gelatinous precipitate. The precipitate obtained with the ammonia is filtered off and a drop of ammonium sulphide added, when any zinc present will be thrown down as white precipitate of zinc sulphide; to the filtrate from this ammonium oxalate may be added, when if lime is present a white precipitate of calcium oxalate is obtained.
A test for iron is to dissolve some of the ash in a little hydrochloric acid and add a few drops of potassium ferrocyanide solution, when if any iron be present a blue precipitate will be obtained.
To make more certain of the presence of chrome, heat a little of the ash of the cloth with caustic soda and chlorate of soda in a porcelain crucible until well fused, then dissolve in water, acidify with acetic acid and add lead acetate, a yellow precipitate indicates the presence of chrome.
A book on qualitative chemical analysis should be referred to for further details and tests for metallic mordants.
The fastness of colours to light, air, rubbing, washing, soaping, acids and alkalies is a feature of some considerable importance, there are indeed few colours that will resist all these influences, and such are fully entitled to be called fast. The degree of fastness varies very considerably, some colours will resist acids and alkalies well, but are not fast to light and air; some will resist washing and soaping, but are not fast to acids; some may be fast to light, but are not so to washing. The following notes will show how to test these features.
Fastness to Light and Air.--This is simply tested by hanging a piece of the dyed cloth in the air, keeping a piece in a drawer to refer to, so that the influence on the original colour can be noted from time to time. If the piece is left out in the open one gets not only the effect of light but also that of climate on the colour, and there is no doubt rain, hail and snow have some influence on the fading of the colour. If the piece is exposed under glass the climatic influences do not come into play, and one gets the effect of light alone.
In making tests of fastness the dyer will and does pay due regard to the character of the influences that the material will be subjected to in actual use, and these vary very considerably; thus the colour of underclothing need not be fast to light, for it is rarely subjected to that agent of destruction; on the other hand, it must be fast to washing, for that is an operation to which underclothing is subjected week by week.
Window curtains are much exposed to light and air, and, therefore, the colours in which they are dyed should be fast to light and air. On the other hand, these curtains are rarely washed, and so the colour need not be quite fast to washing. And so with other kinds of fabrics; there are scarcely two kinds which are subjected to the same influences and require the colours to have the same degree of fastness.
The fastness to rubbing is generally tested by rubbing the dyed cloth with a piece of white paper.
Fastness to Washing.--This is generally tested by boiling a swatch of the cloth in a solution of soap containing 4 grammes of a good neutral curd soap per litre for ten minutes, and noting the effect whether the soap solution becomes coloured and to what degree, or whether it remains colourless, and also whether the colour of the swatch has changed at all.