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100 08 99.88, 100 33 99.94 100 00 100.00 100 44 99.88 99.58 99-99 99-99 99-87

I. Natural hæmatite from near Ulverston.

II. Manufactured Indian red.

III. Natural Indian red from India.

IV. Scarlet red.

V. Purple oxide.

VI. Rouge.

VII. Red chalk from Speeton, Yorkshire. (Clapham, Chem. News, 1862; p. 313).

VIII. Indian red.

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add carbonate of soda until the solution is neutral, then ammonium acetate; boil, filter, wash and dry, and weigh the precipitate; this consists of oxide of iron, alumina, and, in some rare cases, phosphoric acid, but this may be neglected, as a rule, in iron reds. The iron may be estimated in another portion of the original solution by a volumetric test and the amount of alumina calculated from the two results. The filtrate from the precipitate is mixed with a small quantity of ammonium sulphide to precipitate any manganese, this precipitate being collected, dried, and weighed. To the filtrate is added ammonium oxalate to precipitate the calcium, which is filtered off, dried, and weighed. To the filtrate from this, sodium phosphate is added to precipitate the magnesium, if present.

For the sulphate which is present 100 cc. of the original solution are taken and some barium chloride is added; the precipitate of barium sulphate is filtered off, dried, and weighed.

For fuller details as to the method of carrying out this scheme, works on quantitative analysis, such as that of Professor Sexton, published by Griffin & Co., should be consulted.

The analyses on p. 109, which, with the exception of Nos. 3 and 7, have been made by the author, will show the composition of the iron reds in common use as pigments.

A pigment must possess two properties, good colouring power and body. As the iron reds are so variable in their composition it follows that in these two particulars they will vary also; for these two properties they should be assayed, the methods of doing which will be found detailed in the chapter on assaying pigments.

ANTIMONY VERMILION.

This pigment, also known as antimony orange, is very largely used for colouring india-rubber; for other purposes it is not used as extensively as it might be.

It is the sulphide of the metal antimony and has the formula Sb, S. This body occurs naturally as the mineral stibnite or antimony glance of a lustrous black colour; when ground up it is known as black antimony, and is used for various purposes in the arts, one being as a source for the manufacture of antimony vermilion.

PREPARATION OF ANTIMONY VERMILION.Murdoch, in 1847, patented a process for the preparation of antimony vermilion and a similar process was the subject of a subsequent patent taken out by Clark. The pigment can be made in

two modifications, orange and red; the former by precipitation with sulphuretted hydrogen, the latter by other agents.

(a) Orange Antimony.-Murdoch dissolves the black antimony in hydrochloric acid; during the operation some sulphuretted hydrogen is evolved and may be used for precipitating another solution previously made; so as to form a solution of antimony chloride, which is concentrated till it has a strength of 19° Tw.; through it is then passed a current of sulphuretted hydrogen gas which precipitates the sulphide of antimony as an orange powder, which, after being well washed and dried, is ready for use.

Fig. 17 shows a convenient form of apparatus for precipitating antimony vermilion. A is a vessel made of wood, lined with lead, and fitted with a lid which, while being removable to admit of the sulphide of iron used for the preparation of the gas, can

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Fig. 17.-Apparatus for making antimony vermilion.

D

yet, by means of india-rubber joints, be made gastight; B is a funnel for the admission of acid; C is a two-necked bottle filled with water to wash the gas as it comes over; D,D,D, are threenecked bottles (whose construction is shown in the drawing), containing the solution of antimony through which the gas is passed. The most convenient method of preparing the gas is by the action of dilute sulphuric acid on sulphide of iron.

By varying the strength of the antimony solution the shade of the resulting pigment can be varied to some extent; thus, a solution of 19° Tw. gives an orange-red; one of 40° Tw., a reddishorange; while one of 52° Tw. gives an orange colour. When strong solutions are used the precipitate is, however, liable to contain free sulphur, which is sometimes objectionable as it may cause decolourisation of the pigment.

(b) Scarlet Antimony.-1. Mathieu Plessy Process.-This consists in precipitating a solution of chloride of antimony with a solution of sodium thiosulphate (hyposulphite of soda) under

certain conditions. A solution of chloride of antimony of 40° Tw. is prepared. The ordinary commercial chloride is a liquid of about 1-26 (52° Tw.) specific gravity; if this is diluted with water in the proportion of 5 vols. of the chloride to 2 vols. of water, a solution of about 40° Tw. will be obtained. A solution of sodium thiosulphate of 40° Tw. is also prepared; this will take about 14 ozs. weight of the salt to 25 ozs. measure of water.

7 gallons of the thiosulphate solution are taken, and into them are poured 3 gallons of the antimony solution; a black precipitate forms at first, but this disappears rapidly. The liquid mixture is now gently heated; when the temperature reaches about 78° to 90° F. a yellow precipitate begins to form; as the temperature increases the colour changes, passing through various shades of orange til at about 130° to 140° F. it acquires a scarlet-red colour; the operation is now stopped and the mass allowed to cool down; when cold, the clear supernatant liquor is poured off, water containing a trace of hydrochloric acid is poured on to the precipitate, which is stirred up; after which the mass is allowed to stand to settle, the top liquor poured off, and the colour washed with water two or three times; it is now dried at a low temperature (about 140° F.), and when dry is ready for use.

During the process of precipitation a very considerable evolution of sulphur dioxide gas takes place; therefore, the operation should be carried on in a place and under conditions where the gas cannot cause inconvenience.

2. Wagner's Process.-4 lbs. of tartar emetic and 3 lbs. of tartaric acid are dissolved in 18 lbs. of water, and the solution heated to 140° F., a solution of sodium thiosulphate of 40° Tw. added thereto, and the mixture heated to 180° F. The red is gradually precipitated, and when fully formed is washed with water and dried.

In drying antimony vermilion it is important that the temperature be kept low, and not be allowed to rise above 160° F.

The colour of the scarlet antimony powder is rather dull, but it becomes bright when mixed with oil or water.

PROPERTIES OF ANTIMONY VERMILION.-Antimony orange is a light, bulky, orange-coloured powder, while antimony vermilion is a scarlet powder rather heavier than the orange variety. Both pigments have practically the same properties; they are not attacked by dilute acids, but strong nitric acid gradually decomposes them, with the formation of white antimonic oxide and sulphuric acid. Strong hydrochloric acid has little action in the cold, but when boiling

gradually dissolves them with the formation of chloride of antimony and evolution of sulphuretted hydrogen. Caustic soda and potash when boiled dissolves the colour, which is re-precipitated as an orange precipitate on the addition of an acid; ammonia has little action; lime has a similar action to soda.

Both the orange and red forms are rather dull in the pulverulent state, but when mixed with oil or water they become bright; being opaque they have a good body or covering power, and mix well with oil, but cannot be used with alkaline vehicles, like lime or silicate of soda, which have a decolourising action. They are unalterable by air or light, or by deleterious atmospheric agents.

They can be mixed with all those pigments which are unaffected by sulphur. COMPOSITION

OF ANTIMONY VERMILION.—

Pure sulphide of antimony, Sb, S3, has the following percentage composition:

Antimony,
Sulphur,

71.42

28.28

100'00

The antimony pigments are liable to contain free sulphur, especially those which are made with sulphuretted hydrogen. The following is an analysis of an orange antimony:

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from which it will be seen that the sulphur is greatly in excess of that required by theory, so that some of it must be in the free condition.

The red antimony vermilions approach more closely to the theoretical composition, as will be evident from the following analyses :

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the latter analysis by the author is of an English made sample, the other analyses are taken from a foreign work on pigmentmaking.

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