This is an interesting article reblogged with the kind permission of my friend Andrew Sanderson. There are a few different ways of intensifying a thin negative to improve it but Andrew has chosen to speak about thiocarbamide toning in this case.
Andrew is an Ilford Master Printer so really knows his stuff. He writes about photography and techniques at his blog, The Web Darkroom, so please feel encouraged to pay it a visit and leave a comment if you do.
By Andrew Sanderson
We try our best as film photographers to get a correct exposure every time but occasionally there are times when we inadvertently cock it up. It may be because we have forgotten to add extra exposure for bellows extension or a filter factor or the fact that long exposures need a calculation for reciprocity failure.
Sometimes light meters may be set to the wrong ISO/ASA. Then of course there might be a problem in processing with temperature drop, out of date chemicals or just bad luck. Whatever the reason, we have a thin neg and it is a problem to get the tones that we require in the print.
There is a way to rescue these negatives though: they can be chemically enhanced to increase density and this is most easily done by toning the negative, giving a brown colour to the negative image. The brown image blocks more light for blue/green sensitive papers so the negative is effectively darker and denser and this prints lighter.
To illustrate this technique, I took a second rate 10×8 negative and chopped it in half. I bleached one half in the standard sepia toner bleach bath (formula at the bottom of the article) until all the silver had gone. This left me with a pale pink negative.
After a ten to fifteen minute wash, I immersed the bleached negative in the toning solution and in less than a minute it had gone a deep brown.
I then washed it for twenty minutes and hung it from one corner to dry. I contact printed the two halves on one sheet of 10×8 paper to see the comparison and this was the result;
You can see quite clearly that the right hand side is much improved.
Some darkroom workers tone their negatives using selenium rather than sepia and this works just as well (when using selenium, the bleach does not need to be employed).
The bleach is made up of two chemicals; potassium ferricyanide and potassium bromide. Measure 100 grams of the ferricyanide and stir into 1 litre of water. When it is all dissolved, add 50 grams of potassium bromide to the solution and stir until that is also completely dissolved. The bleach is ready to use immediately and works quite quickly.
The toner I used was a thiocarbamide toner, though a sulphide toner works just as well. This is made as two stock solutions which are mixed together before use.
Stock A. Water 1000 ml, thiocarbamide 100 grams.
Stock 2. Water 1000 ml, sodium hydroxide 100 grams.
Take 200 ml of each and mix together, immerse the bleached print and gently agitate. The print will be toned within a minute.
Wash negatives for 20 minutes and hang up to dry. The mixed solution can be kept for months if tightly stoppered and stored in the dark.
If toning prints, wash for 20 minutes (RC), or 40 minutes (FB).
Here's the link to the original post at Andrew's blog.