Sensitizing gum arabic coatings.

Hello again,
Since I am not a gum printer and do not use gum arabic I cannot say I have any real experience of the following process and quite honestly I do not wish to get all gummed up with it, at present. But, there may be a few hard liners out there who would like to give it a go and keep the pot boiling.

I strongly suspect that the vast majority of gum printers mix the dichro in with the gum/PVA/ gel/ pigment, whatever, and then brush or roll on the coating.

         Came accross a method in Clerc (pub 1954) whilst going thru' the chapter, again, on Pigment Processes called ''Bichromate Wash-drawing'. The most relevant bit is as follows'....
''The sensitizing is done, as required, by immersion in a 1% solution of ammonium bichromate, with the addition of 1% of neutral sodium or potassium citrate.''
    
 And what coating are we sensitizing ?

 ''Now, put 1g of degreased lamp black on a piece of glass placing the powder in the form of a crater. Then pour into this 2.5 ml of a 50% solution of Gum Arabic; mix thoroughly with a flexible knife, adding gradually 2 ml of water. Grind with a glass muller etc.,.....
To cover a sheet of paper 16'' X 20'' about 2gms of this paste is placed in a clean cup, and about 0.75'' of the strip delivered by a tube of sepia water colour is added. This gives a warmer tone, and, at the same time improves the adhesion of the coating. Now add from 15 to 18 ml of the following mixture, prepared with boiling water -

Crystallized sugar 2gms
Starch 2gms
Water 100 ml.

After mixing thoroughly.....etc., etc., ''

Coating is done with goat hair and swallow tail brushes.Dry thoroughly. After UV exposure the print is immersed face downwards for about 10 minutes in a dish of water at 115 F and the print moved about from time to time. The print is then laid on a slope and then cleared/developed by spraying with water at 18 F above that of the soaking water.

I could do a verbatim copy of the entire process, later and put it on the List, or post only to those interested. Just let me have your request.
           Happy times.
    John Grocott- Photographist - London
SYPE (video) name CARBONS999
Received on Wed Apr 5 15:39:16 2006