My trusty Iliffe Dictionary of Photgraphy tells me that it, or Cutch to
give it another name,is a brown gum rich in tannin used for toning platinum
prints. Half an ounce of cutch in 10 oz water, ie a 5% solution. is boiled
and 2 oz alcohol are added. This is the stock solution. For the toner take
1 drachm with 20 oz of water and heat to 150 F. The platinum print goes a
rich brown after 15 mins. If the temperature is below 130F it will take
longer. Wash in plain cold water to stop the toning action. Browner tones
can be got by adding 20 drops of 10 % ferric oxalate. Degraded highlights
can be cleared in 1 % potassium dichromate after which the print is
thoroughly washed..
Terry King