C₁₆H₁₄O₆ + NH₃ + O = C₁₆H₉O₅.NH₄ + 2H₂O.
To obtain hæmateïn in the pure state it is then only necessary to add sufficient acetic acid to decompose the hæmateïn ammonia compound. Hæmateïn separates as dark violet crystals, which readily dissolve in water and alcohol; its aqueous solution gives blue precipitates with the majority of the metallic salts. This behaviour of hæmatoxylin towards ammonia explains the increase in colouring power of logwood, which has been exposed for a long time to the action of the air in the rasped state. Through the action of the ammonia in the air a larger quantity of the hæmateïn ammonia compound has been formed. It has been proposed to facilitate the formation of this ammonia compound by moistening rasped logwood with a very dilute glue solution and allowing it to lie in the air. This process can only have the object of utilising in the formation of hæmateïn the ammonia resulting from the decomposition of the glue, but in this decomposition deep-seated reactions occur, which might affect the hæmateïn itself. It thus appears more suitable to effect the formation of hæmateïn by the direct use of ammonia. This can be done with little cost by watering heaps of rasped logwood with ammonia and repeatedly shovelling about the wood so that it comes into contact with the air. The author has found that the conversion of hæmatoxylin to hæmateïn is very complete in this process; care should be taken not to make the layer of rasped wood too deep, and to take its temperature frequently. In the transformation of hæmatoxylin to hæmateïn the temperature rises, the rise might be injurious if it proceeded too far. Thus if the temperature of the interior of the heap is found to be high the wood should be turned over.
The solution of the logwood colouring matter produces handsome lakes, all of which have, however, the inconvenient property of acquiring an ugly grey colour on long standing. The finest and most durable of the logwood lakes is known as violet lake, which is made with alumina salts. The best result is obtained when a solution of aluminium acetate, obtained by precipitating alum with lead acetate, is mixed with a logwood decoction or a solution of logwood extract. The precipitate is pale or deep violet according to the amount of aluminium salt added. After drying to a certain point at a gentle heat it can be mixed with gum solution to a paste, which is then completely dried.
Logwood is most valuable in dyeing and calico printing, in which it serves to produce a fine and durable black. When potassium chromate is added to a decoction of logwood a deep black liquid results, which can be used as a good and cheap writing ink. If somewhat stronger solutions are used a greenish precipitate first separates, which soon acquires a pure black hue; it is the chromium lake of hæmateïn. This compound is very durable and is largely used in dyeing to produce fast blacks. The black precipitate might be dried and used as an artists’ black pigment if the carbon blacks were not cheaper and more durable.
CHAPTER LXIII.
GREEN LAKES.
The yellow colouring matters produce green compounds with copper salts. These are important because they can be produced without great cost, and are not as poisonous as the arsenic pigments, although copper compounds are far more poisonous substances than should be used for certain purposes. Their employment might be attended with danger if used to colour children’s toys and pictures.
These green copper lakes are simply made by adding copper sulphate solution, free from iron, to a hot decoction of yellow berries or weld until the liquid is emerald green, and then adding caustic soda solution in small quantities to precipitate the lake. The temperature of the liquid should not be more than 50° to 60° C. when the caustic soda is added. It has been maintained that these lakes turn out well only when the precipitation is so conducted that the residual liquid is quite colourless. The author has, however, found it more satisfactory to discontinue the addition of caustic soda when the liquid is somewhat coloured, since when the precipitation is complete other substances besides colouring matter may be precipitated.
Chlorophyll.—All the higher plants contain the same green colouring matter—chlorophyll. It has a very fine shade, is fairly fast, and the raw material from which it is produced is to be obtained at nominal cost. Special attention should, therefore, be paid to this colouring matter.
Chlorophyll is very easily obtained. Grass of a good green, green leaves, or any green portion of a plant is allowed to stand with weak caustic soda solution in a large vessel for twenty-four to thirty hours. The liquid is poured off, brought to the boil for a moment, at once filtered and neutralised with hydrochloric acid. The chlorophyll, which has been dissolved by the caustic soda, is then thrown down as a grass green precipitate; after washing and drying it can be used as a pigment. A chlorophyll lake can be obtained by dissolving the colouring matter in caustic soda and adding alum solution. The green precipitate is a compound of chlorophyll and alumina.
Tschirch gives the following method for obtaining chlorophyll: Green leaves (grass), not containing tannin, are extracted with boiling alcohol, the extract cooled, filtered and evaporated until a sticky mass remains. This is washed with hot water until the washings are colourless, and the residue extracted with cold alcohol. The solution is evaporated to half its volume, the separated crystals are dissolved in alcohol, and the solution heated on the water bath with zinc dust. A deep emerald green solution with a red fluorescence is obtained; it can be preserved unaltered for a long time in blue glass bottles. It retains its colour in diffused daylight also for a lengthy period. A beautiful green chlorophyll lake is obtained by boiling this solution with the solution of an aluminium salt and precipitating with soda. Unfortunately, the lake is not fast. The chlorophyll solution obtained by this process is quite innocuous, and is well adapted for colouring liqueurs and confectionery.