[alt-photo] Arabin Gum Process -- Not Arabic.

[Richard Sullivan]

ARABIN GUM BICHROMATE PROCESS 

A gum bichromate printing process worked out by Nelson K Cherril and
published in June 1909 To prepare the arab1n a quantity of best Soudan gum
arabic is sifted through a 40 mesh sieve Place in a quart earthenware jar 1
50 ccs of water 7 J ccs of pure hydrochloric acid and then sift into the
mixture stirring the while 100 g of the powdered gum Keep the whole at about
1200 F about 49 C and stir frequently until solution is complete Cool and
add 600 ccs of the best methylated alcohol free from petroleum and stir for
half an hour or so or until the arabin is thrown down as a white precipitate
and has lost all stickiness or gum miness Filter through two thicknesses of
cheese cloth gather the arabin in the cloth into a ball and squeeze it well
place it in a small jar cover with new spirit stirring it and breaking it up
well and leave for several hours until the spirit has absorbed all the water
Squeeze again in cheese cloth then put the arabin in a towel and squeeze it
in a press with as heavy pressure as possible Break up the cake formed to
allow the remaining alcohol to evaporate with gentle heat then break the
remaining lumps in a mortar and dry until all is a dry gritty powder The
formula in the English system would be roughly as follows:

Water 5J oz 
Hydrochloric acid 127 mins 
Powdered gum 1540 grs 
Methylated alcohol 21 oz 

To prepare synthetic gum take 20 g of Arabin, 20 g of heavy magnesium
carbonate from 40 ccs to 75 ccs of water according the thickness of the
solution preferred. 

This formula in English is Arabin 308 grs 
Magnesium carbonate 308 
Water to 1 1/2 ox to 2 1.2 oz mixed stir occasionally until the froth then
filter through muslin

To prepare the pigment lampblack is used; Wash it with repeated doses of
mixed ether and acetone until all fatty gummy matters are removed or
preferably burn small pieces of camphor slowly under a piece of porcelain --
say bottom of a porcelain developing dish. Scrape off the soot with a
palette knife into a test-tube and wash with mixed ether and acetone until
these solvents come away with slight discoloration. Pour off as much
possible without losing the black and dry stirring the test tube in hot
water keeping water from the pigment. When dry, the tube is inverted and the
black will fall out freely. special lampblack known as No 4 has been for
this particular process.

To mix gum and pigment for coating upon paper, it is necessary to experiment
with the particular paper to be used taking a normal temperature say 95 Deg
F for the developing water and a normal time say forty five minutes for
development. The mixture must be such as will just soak clean from the paper
in the development time. With too little gum, the pigment soaks into the
paper, with too much it washes away before development is complete. In
practice it is best to make up an under-gummed and an over-gummed ink and
experiment with these will show the proportions for any paper. For instance
Cherril recommends the making of one ink containing gum in the proportion of
20 arabin to 75 water and the other in the proportion of 20 to 45 of water.
If both these are pigmented in the same proportion as to quantity that is
proportion quantity that is to say about 400 to 500 mm of lampblack to each
10 ccs the one will be found to give too much penetration to Joynson's or
Rive's paper and the other too little a mixture of the two will be found to
give a good result The mixture is sensitised just before use by an addition
of an equal volume of bichromate solution made by adding 15 gm (230 grs) of
ammonium bichromate to 100 ccs (3 1/2) oz of water dissolve by heat and
neutralise by stirring in a little chalk decanting when effervescence ceases
and the solution settles. The paper to be used is brushed over thinly with
the freshly mixed gum and bichromate the brush marks being obliterated by
crossing and recrossing the strokes. After drying the paper is ready to be
exposed. Exposure should be by actinometer after the manner of carbon and
the paper is much more sensitive than the average gum bichromate paper
prepared by other processes. If the development of a print from an ordinary
negative is complete in about forty five minutes in water at a temperature
of 95 Deg F (35 Deg C.) the result will be perfect. Development may be
performed in a vertical tank or by floating face downwards on the water
controlling in the usual manner.