Iron is the most important structural material used in dye chemistry, and is utilized in every variety and form.

In the form of cast iron it is used for sulphonating- and nitrating-pots, for evaporating plant, cocks, stirring-gear, autoclaves, and, in short, wherever the liquids dealt with are neutral or alkaline. The insufficient tensile strength of this excellent and easily cast metal alone prevents its still wider use.

As is well known, the properties of cast iron vary considerably according to its chemical composition. For acid-resistant cast iron, that is to say, such as is little attacked by concentrated acids, ordinary grey cast iron is made use of, its resistance being improved by certain additions which are kept a secret by the various foundries.

Ordinary grey cast iron answers all requirements when dealing with sulphuric acid of at least 75 % strength, nitric acid, or mixtures of the two. It becomes passive, and so acquires quite a fair resistance even to moderately dilute acids. It never does, however, to trust to luck in these cases, and experiment alone can decide whether grey cast iron will do in a given case. Further, the vessel must be carefully cleaned out after every stoppage. Grey cast iron vessels must be washed out when a manufacture is stopped, traces of acid removed with boiling soda solution, the washing water blown out at the boiling-point, and the small remainder swabbed out so that the vessel is completely dry. If the pot stands in a wooden water-bath, then the latter must be kept filled to prevent shrinkage, and the water should be made strongly alkaline by means of soda to prevent it becoming foul.

Further, the agitator brackets of vats, autoclaves, and other vessels are constructed from grey cast iron. To ensure easy running the toothed wheels should be well oiled, and it is also very advisable for all larger pieces of apparatus to be mounted on ball-bearings whenever possible, as by this means a considerable saving of power and lubricant is effected. The stands and end-pieces of filter-presses are made of cast iron, but not the tie-rods, as cast iron has not sufficient tensile strength for this purpose. Autoclaves may be made from cast iron for working up to 40 atmos., but for higher pressures cast steel must be used, as cast iron is liable to contain blow-holes when used on too large a scale and, further, the walls required would be far too thick. The autoclave shown on Plate XII (Fig. 32), constructed of cast steel, has walls 80 mms. thick, and weighs 10 tons; to construct a vessel of similar capacity with a diameter 1.2 metres of cast iron, and capable of withstanding a working pressure of 40 atmos., it would be necessary to make the walls 400 mms. thick, and its weight would be over 60 tons. Such a freak apparatus could not in any case be used technically, owing to the enormous tension which would be produced on heating. The fusion-pots for the manufacture of naphthol are also made of grey cast iron, and it has been found that the addition of 1-3 % nickel increases the resistance to alkali to a remarkable extent; fused alkali, especially caustic potash, attacks iron very strongly.

A type of cast iron which is completely, or almost completely, unattacked by acids has been on the market fairly recently in the form of alloys containing about 12 % of silicon and 4-6 % aluminium. This ferro-aluminium silicon alloy is, however, somewhat strongly attacked by hydrochloric acid; it was first made use of in England under the names of Ironac and Tantiron; there are also certain imitations known as Kieselguss, Azidur, and Clusiron, which can all be cast very easily but are unfortunately glass-hard and brittle, so that they have to be worked with an emery wheel. For nitric acid distilling plants these alloys serve excellently, and they also form a welcome addition to the list of materials available for various other special purposes, but they cannot be used for the linings for autoclaves owing to their brittleness.

Where great strength is necessary, wrought iron, ingot iron, and steel must be used. These forms of iron are used for the tie-rods of filter- and hydraulic-presses. The head-pieces of the latter must be made of cast steel, as cast iron has not sufficient strength. In recent years also Swiss electric steel has been used. Steel is also used for the spiral tubes of spring manometers where ammonia is dealt with. Ingot iron is employed for the hoops of vats.

Formerly, rather more use was made of copper than at present, but even to-day it is quite indispensable. It is used for scoops (but not for ordinary diazotizations), for the baskets of centrifuges, for piping, and, in particular, for drying trays, where it is used almost exclusively. It is not resistant towards ammonia admixed with air, and is often tinned in order to protect it. Alcohol stills are usually made of copper.

Tin is hardly used at all as such, but only in the form of alloys, such as bronze, lead-tin alloy for filling baths,1 and, especially, for tinning iron and copper vessels 2 (see homogeneous lead coverings).

Zinc, also, is rarely used as such, but chiefly in the form of brass and bearing-metal alloys, and also as the coating of the so-called "galvanized iron."

Aluminium, on the contrary, owing to its great resistance towards dilute and concentrated nitric acid, is coming more and more into use. It is frequently met with in the form of piping for nitric acid and for nitrating pots, but it has the disadvantage of offering only a poor resistance to factory air.

Nickel is hardly ever used except in special alloys.

Of other metals than iron, lead is by far the most important, and is quite indispensable. It is found in nearly all filter-presses in the form of lead tubes, and also for other piping which has to deal with acid and alkaline liquids. The head-pieces of the filter-presses are covered with sheet-lead, as also are the inlet tubes. It is often found on heating metal which has been covered with lead that the latter becomes loose, develops large blisters, and finally breaks away. This disadvantage is overcome by fusing or alloying the lead covering to the metal beneath, instead of merely laying it on. Apparatus which has been covered with an intimate coating of lead in this way is said to be homogeneously lead-lined; this homogeneous lead covering is playing an increasingly important part in colour technology. Circular apparatus such as the lining for autoclaves, and so on, is lead-lined according to the method of Kuhnle Kopp and Kausch by rotating the vessel rapidly and then pouring in lead. In this way all the pores of the metal are completely closed, and it is possible to deal with plant up to 6000 litres and weighing up to 10 tons. Iron and copper, before being treated in this manner, must first be tinned, otherwise the coating does not adhere well. This layer is often quite thick, 2 mms. and more, so that several thousand kilograms may be required for a large piece of apparatus.

1 An alloy made with equal parts of lead and tin does not expand at all, practically speaking, on heating.

2 The inlet-pipes of filter-presses, and also the cocks of colour vats, are practically always made from the best quality bronze.

These short notes do not, of course, in any way exhaust the uses of metals in the dye industry, but they suffice to show what a large part is played by these structural materials in the industry.