Post by Romeu Trauer
Foxing is that reddish-brown stain that appears on stamps. In reality, these are funghi that attack the paper’s cellulose fibres. Methods for getting rid of foxing are known for quite some time. Here, we shall use the publication of the Sociedade Filatélica Paulista from 20 of December of 2008 (by Sérgio Marques da Silva) as a reference.
The best known methods are those using potassium permanganate or chlorine-based products. With the new bleaches that do not contain chlorine, other ways of dealing with foxing have also been tried out.
1 – Potassium permanganate method
Potassium permanganate has its molecular formula KMnO4 . In its solid state, it presents itself as purple-bronze crystals and its solubility in water is 7g in 100ml (meaning 100%) at 25 degrees Celsius. With this proportion, the solution is saturated. If we use 2 grams in 200ml water, we should therefore have a solution of about 14%. This is sufficient for our experiments. The amount of time the stamp should remain under the solution varies on the degree of foxing contamination but, in general, 5 minutes should be sufficient. Using a higher concentration, the stamp should be dipped for less time.
After dipping, the stamp should be taken out of the solution and put into a clean water tray to remove the excess permanganate.
Subsequently, the stamp should be dipped in a solution that neutralizes the permanganate. It can be noted at this point that purple spots can be found where foxing was.
Two processes are most used:
1.1 Using citric acid or lemon juice
1.2 Using sodium bisulfite.
1.1.1 – Using lemon juice.
After showering the stamp with P.P., the stamps will be rather dark. After a short dip in water (in a separate tray), we place the stamp on a third tray, containing half a litre of water and the juice of half a lemon (which contains citric acid) and leave the stamp in the tray for about 3-4 minutes.
After this time, the stamps should already have gained back their natural colour. After that, dip the stamps in clean water again (from a fourth tray) and leave them to dry on kitchen paper.
1.1.2 Using citric acid
The formula for citric acid is C6H8O7 and its solubility is of 133 grams in 100 ml of water. So, for us to obtain a solution of 10% in 200 ml of water we need about 26 to 27 grams. This corresponds to about a coffee spoon of the product. The treatment is done the same way as with the lemon juice.
1.2 Using sodium bisulfite .
Sodium bisulfite, NaHSO3, is a white powder in commercial form. It is usually used in wine to prevent oxidation and preserve the flavour. We can use it to neutralize the effects of the potassium permanganate that remained on the stamps. We can use it the same way as described in the lemon juice section. The solubility of sodium bisulfite is of 42 grams per 100 ml of water, meaning a solution of about 20% requires 8.4 grams in 100 ml.
I researched the reasons to use sodium bisulfite in stamps and I found that when it is mixed in water, the sulphite radical SO3 oxides and leaves the sulphate SO4, which neutralizes the potassium permanganate, and acts like a whitener. Therefore, if the stamp is only a bit yellow from the years or with small hinge marks, a bath in a 10% solution of sodium bisulfite and water should help whiten the paper. (Stockflecke in Grosses Lexikon de Philatelie - Ullrich Häger - Goldmedaille IBRA'73).
It is of great importance to wash the stamp in water afterwards. A last bath with salty water is also recommended to revive the colours.
The main element here is sodium hypochlorite (NaCIO). The Na+ and CIO- ions are separated when mixed with water. Depending on the concentration, it has a variety of uses. In contact with the skin, it can leave burns.
2.1 Bleach has on average 2 % of active chlorine. Its pH is over 12, highly alkaline. Two or three drops in a glass of water may be used as a solution to fight foxing in stamps.
2.2 Dakin’s fluid is a solution made of sodium hypochlorite, sodium bicarbonate and water. It was invented by the British chemist Henry Drysdale Dakin and the French surgeon Alexis Carrel. It is used in medicine as an antiseptic. It is usually sold in pharmacies where dental products are sold (Dakin’s fluid with 0.5% of sodium hypochlorite).
Placing the stamp into the solution allows us to get rid of foxing.
2.3 Food detergents (example used: Brazilian brand Hidrosteril) is a product similar to the others and used to wash fruits and vegetables. Every ml of Hidrosteril contains 8 mg of active chlorine, 0.1 mg of potassium permanganate, and 1ml deionized water. For washing stamps, three or four drops in 100 ml of water should be enough to resolve foxing.
3.1. Vanish Powder O2 Max .
Liquid Vanish uses hydrogen peroxide instead of chlorine. This way, the product does not violate paper fibres the same way chlorine based products do. Vanish solution is very alkaline, with a pH over 8. Solid vanish should also work, and its composed by sodium percarbonate, which aids dissolving stains.
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