On occasions I find that some of my gum prints are stained yellow, which does not come out with washing. This is due to the dichromate being trapped in the paper or somehow attached to the surface of the paper. This is a well recognised phenomenon and the recommended treatment is to clear it with sodium metabisulphite.
The chemistry behind this involves the sodium metabisulphite reducing the chromium from its hexavalent state Chrome(VI) to the trivalent Chrome(iii).
Sodium metabisulphite is used in many households as an antimicrobial in home brewing and wine making - it is readily available. However, although it is not as bad as the dichromates, it still needs to be treated with respect - here is a Safety Data Sheet I've downloaded from the internet. While it is safe I did find when weighing out the powder and then when spraying the solution I preferred to wear a mask.
As with any alternative printing processes there is some discrepancy in the detail of how to do this clearing. There seem to be two main approaches before a final wash:
- Soaking in a bath of sodium metabisulphite solution
- Spraying the sodium metabisulphite solution directly onto the print
Of the two I much prefer to work with a spray. My prints are large (760mm by 560mm) normally and any bath will require a large amount of solution.
Then there is the question of concentration - I am using two sources Christopher James' The Book of Alternative Photographic processes and The Carbon Print by Sandy King and John Lockhart. James recommends a 1% solution whereas King & Lockhart use a 3% solution.
Since the metabisulphite is not expensive and readily available I would prefer the stronger solution - 3%mass/volume.
There is further confusion as to what % means when applied to solutions and I am going to adopt the convention of % mass of solute in grams (g) / volume of solvent in millilitres (ml) or %mass/volume. Here the solvent is water, which at 0 Celsius has a density of 1 g/ml so its is almost equivalent to %volume/volume. But I will be using other liquids so my convention is to stick with %mass/volume.
I have some wide-mouthed storage jars that hold 500ml, so I am going to fill one of those and store what I don't use now for future use.
The calculation is simple:
3%mass/volume = 15g sodium metabisulphite to 500ml deionised water
This mixes easily and I transferred to the spray bottle using a funnel
The print I used had an unsightly stain over most of the prepared area - I was able to spray the whole area easily. I continued until I could see the whole surface fully wet. In about a minute the staining disappeared. I then washed the print in warm water for about 5 minutes.
Hanging it up to dry I was able to inspect it in detail - I am very happy with the result.
- This clears instantly and is fun to watch
- I found that spraying the metabisulphite made me cough, so it is important to use a mask