Pigments, Paint and Painting: A practical book for practical men

by George Terry

When a chamber is prepared for the converting operation, the whole of the lead it contains will be in metallic communication with the tin supports, and these with the tin covered bottom of the chamber.

The chamber when working is kept at a certain temperature by a steam coil beneath the floor of the chamber. The process is simple. The lead buckles or gratings are placed on tin-covered stands, somewhat in form and make like a dinner-waggon. The whole is hauled up and dipped into a bath of acetic acid and acetate of lead; it there remains for one or two minutes, it is then hauled out, drained and lifted bodily through the top into the chamber. Other stands filled with buckles are so dipped and so placed till the space of the chamber is fully occupied.

This dipping cleanses the surface of the metal, and when it is exposed to the air it is speedily coated with a hydrated oxide of lead. This is the first step in the process of conversion. The chamber when filled is closed, and its temperature is brought to about 100° F.; then vapour of acetic acid and vapour of water and air are supplied from without to the interior of the chamber. This is continued for 15 or 20 hours. The lead buckles within the chamber will now possess a whitish coating, consisting of subhydrate and subacetate of lead, and they will present a uniform colour. Carbonic acid generated in any convenient manner is next passed into the acid generator; it mixes with the other gases and vapours, and with them goes on its way to supply the chamber. Speedily the action of the carbonic acid is observed, the surface lead becomes quite white and presents the appearance of a snow shower having fallen within the chamber. The formation of white lead is now speedily effected. This treatment is continued throughout the space of 13 days; at the end of this time the supply of acetic acid vapour is stopped, and the supply of air, steam and carbonic acid is continued, according as it is desired to obtain white lead rich in oxide or in carbonate.

After a short further period, steam and air only are sent into the chamber, which is varied in temperature to 120° or 130°F., and lastly the steam supply is stopped; air alone enters the chamber, which is kept heated by the coils beneath the floor. The contents of the chamber are now in a dry state, and the operation is terminated.

It will occur to most readers that these terminal proceedings amount in effect to a convenient method of washing and drying the white lead while it is still attached to the parent lead, and this it is in fact.

The contents of the converting chamber are lifted out through the opened top, and the buckles or gratings with their crust of white lead are turned into the agitator. This agitator is an iron cage revolving inside a closed chamber of the same material. During the revolution of this cylinder or cage, the contained lead gratings fall from side to side, and the incrustation on their surfaces becomes detached and broken up. It falls in, this broken state through the bars of the cage or cylinder into a receptacle beneath. The denuded buckles or gratings are retained in the cylinder and are removed. These gratings or buckles are cast of such a thickness as to withstand two or three converting operations in the chamber before they are recast.

This crude white lead is carried by an elevator, or it falls into the hopper of a pair of granite crushing rolls, also enclosed; and from these it passes into the mixer or incorporator from which it can be removed in a dry state or mixed with oil.

The incrustation of white lead will be found upon examination to be possessed of some peculiarities, the result of the electrical action which has been going on within the chamber. It is quite white. It falls from the lead buckle or grating which it coats, if the grating be struck against a piece of wood with but a slight blow. It is easily friable, and can be rubbed to the finest powder between the thumb and finger, or on the palm of the hand.

Now, we may explain, as we conceive it, the philosophy of its production in this state of disintegration.

We know that a feeble and prolonged current of electricity will in time deposit metals from their solutions in a crystalline condition, and that if we quicken the current of electricity and cause it more energetically to act on the same solution, we can precipitate the metal from that solution in a state of powder.