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Perhaps because they are interesting freaks, chemical changelings occupy an important niche and are popular in philatelic circles, more seriously from the angle of fraud. A detailed background to this phase of philatelic knowledge is given here.

Printing inks are made from colouring material mixed with varnish and oil. The colours used may be divided into two groups: organic, including the coal-tar dyes, and inorganic or mineral, including the ochres, chrome yellow, ultramarine blue, white lead, and carbon-black. There are probably several hundred different dyes and pigments presently in use, or have been used by the various stamp-issuing countries. Indeed the combinations to produce shades, hues and tints must be in the thousands. Many of these combinations produce colours that cannot be easily distinguished by the naked eye in sunlight. However, they respond to different chemical reagents and to different types of light. The principle of ultra-violet lamps, used to detect repairs, uses different types of lights to clean cancels or forgeries. This lamp, although a valuable addition does not solve all the problems and we could extend the principle greatly through further study.

We could group chemical reactions upon stamp colours into six reactions; namely, acidification, alkalinisation, oxidation, reduction, irradication, and sulfuretting (commonly, but incorrectly, called oxidation). When we consider hundreds of chemicals and thousands of mixtures, obviously we may expect many exceptions. To name a few, the inorganic or mineral group is less susceptible to change than the organic group. Black is usually very stable as it is likely to be compounded from carbon-black, one of the most inert inorganic pigment. Blue is fairly stable and purple is the least. 

Acidification will dissolve many pigments of both groups. In mixed pigments (which includes almost all of them) one may dissolve out and leave other components unaffected or less affected. For example, dilute hydrochloric (muriatic) acid will change many greens to good blue by dissolving out the yellow (chrome yellow) from the original blue-yellow mixture. However, there are some true green pigments for we cannot apply this principle. When using this procedure, the current U. S. purples become a rich lake-red.

Alkalis are the opposites of acids and when added to acids, according to known chemical formulae and in the right proportions, neutralise them to produce a salt and water. For example when sodium hydroxide (an alkali) is added to hydrochloric acid, the solutions are neutralised and what is left is common table salt (sodium chloride) and water. 'Washing soda, and similar proprietary cleaners, liquid ammonia and even soap have alkaline properties to a greater or lesser degree. Generally speaking, a short application of dilute alkali will have little effect on the colour. Usually extended application will result only in a dulling or greying. However chrome yellow is affected. An attempt to wash some dirty 1c '69's, through adding ammonia to the water, caused the design on the stamp to practically disappear.

Oxidation is a bleaching action. For many years a bath with potassium permanganate followed by another bath with oxalic acid was recommended for stamp cleaning. This action bleached the paper, since potassium is a strong oxidizing agent, but in many cases had a bad effect on the colouring matter. Since the paper was stained a dark brown by the treatment and the changed colour would not be apparent until the brown was removed by the second agent - the oxalic acid - the ill effects could not be discovered until too late. A better bleach would be a solution of sodium hypochlorite, popularly used by laundries under the name of Javelle water and lately marketed under the trade name of "Chlorox" or "Oxol", available at most grocery stores. This solution is used by paper mills to whiten paper stock and will restore yellowed paper to white. However, we would need to use a very dilute solution and the stamp must be watched carefully, otherwise a the result will be a deathly white. After removal and placing in water to wash, the stamp will continue to be bleached for a period by the hypo-chlorite retained in the paper fibres. Rinsing and immediately immersing the stamp in hydrogen peroxide, however, can control this. This common antiseptic kills the action just as the hypo solution stops the action of the developer in the photographic technique. However a word of caution: this process is hard on most greens. Indeed it appears that almost anything will turn many greens into blue! Also badly affected are the organic pigments. The dark or lake reds will fade quickly.

We seldom experience reduction reactions on stamp pigments. The general effect is to change the organic dyes to their colourless bases and in the process destroying the colour. Loss of colour by mildewed print is probably an example of this.

Exposure to light is a form of oxidation where some of the rays in sunlight quicken the action of the oxygen in the air. The most common examples are the faded colours in chromos hung on the wall for long periods, or more particularly in stamps mounted in frames for display. In general, the inorganic pigments arc only slightly affected, while the organic dyes lose their colour. We frequently see the 3c and 6c of the 1890 group thus altered. The 6c Columbian is another and there are many $4.00 Columbians ruined by such exposure floating around. A few years ago some excitement was raised when a number of 5c Ericssons were found in a beautiful azure blue shade. It is understood that these were produced by unintentional exposure to sunlight.

Sulfuretting must be distinguished from oxidation. The use of the latter term is a misnomer. Sulfuretting results from exposure to hydrogen sulphide, or sulfuretted hydrogen, which is a gas produced by the incomplete combustion of materials containing sulphur; for example, coal. Hydrogen sulphide is present in the atmosphere in small quantities, especially in the winter. The gas is also formed by the rotting of organic matter containing sulphur in the absence of air or oxygen. In fact it is this gas that gives rise to the sickly smell of rotting carcasses and rotten eggs. Hydrogen sulphide combines with the salts of most heavy metals such as silver or lead to produce intensely black "sulphides." The tarnishing of silver is a common example of this. Chrome yellow, which chemically contains lead chromate, and any pigment containing it, or any other lead salt (basic lead carbonate or white lead is common) will darken if properly contracted. The 3c '51's and the orange revenues are typical. The gas is quite soluble in water. Hydrogen sulphide solution should be available from any school science laboratory. To make the solution, add a few drops (very few) of ammonia to 100 cc of water and add the hydrogen sulphide gas. Almost any yellow or orange stamp, and many bright reds and greens, placed in this solution will turn black practically indistinguishable from correct blacks. As a matter of fact, the colour of practically any stamp will be affected adversely, since small amounts of some heavy metal impurities are almost always present in pigments. If we subsequently immerse the in hydrogen peroxide, the original colour will be restored without any damage. The explanation is simple. The black lead sulphide is oxidized and turns into lead sulphate, which is white and therefore does not interfere. This is the basis on which oil paintings darkened by age are restored. The hydrogen peroxide test is a good one to make on a rare black stamp that has a collared counterpart.

One other type of changeling should be mentioned here. If paper is exposed to a very strong alkali, say a 25 per cent solution of lye, it shrinks. This process is what causes wool to shrink in soapy hot water. To overcome this, after treating a stamp with alkali wash it in water containing a small amount of acetic acid. Ordinary vinegar, diluted 30 to 50 times will do the trick. The end result is almost unbelievable.

The speed at which a chemical change takes place (called the reaction rate) varies with the concentration of the active substance and with the temperature, and the rates for all reactions are not each affected equally. Therefore, although a stronger or warmer solution causes a faster action the end results may not be the same if there are two or more reactions in the change. For example, the alkali of the bleach and the bleach itself may act independently on say three components of the colour mixture and this will give six independent rates. Raising the temperature or concentration will not affect all six by the same amount. Therefore, the final results will be different in the two cases. The caution here is that we must not throw the chemicals together haphazardly. We should exercise qualitative control in order to be able to reproduce the results.

It is thus obvious that any colour is subject to change in a chemical reaction. Therefore, if we wish to make interesting displays, we can get good results with just a few inexpensive chemicals available from most school science laboratories. A little knowledge of chemistry helps. The items are not usually on sale but who knows what close liaison with a school science teacher can bring about. After all, we are not dealing with dangerous chemicals here.