In simplest terms a stain is simply a discolouration that can be obviously distinguished from the surface or material on which it is found. While in some cases, stains are intentional – such as in biochemical research (bacteria-staining) and art (eg. wood staining) – stains in the domestic arena are usually the result of unfortunate accidents and the source of irritation and additional laundry work!
A stain can be formed in 3 ways:
- The substance which spills onto the material is physically trapped by the weave or fibres and the chromophores in the substance (the molecules which absorb light at certain wavelengths, causing “colour”) allow us to see the colours of the substance, ie. we see a “stain”.
- There is a molecular attraction between the particles in the substance and the particles of the material, so that they are held together in an electrical bond (which can be easily broken); however, it is still the chromophores in the substance that cause the actual “staining colour” that we see.
- There is a chemical reaction between the substance and the material, so that they form a new compound – in this case, the stain is often permanent as the chemical bonds are very difficult to break.
In most cases, stains are due to (1) and (2), very rarely (3) as most materials (such as the fibres in carpets) are fairly inert and will not react. However, heat can often change the properties of substances, making them more reactive – thus, this is why it is advised never to dry your clothes in the dryer or use an iron, until the stain has been completely removed as the extreme heat can cause a chemical reaction on an otherwise removable stain, turning it into a chemical compound that is impossible to remove.
For stains that are due to (1) and (2), it is usually enough to add a suitable solvent which will lift off the substance from the material (ie. break the weak molecular attraction between them or enable the substance to escape from the physical trap) – and this is how most stain removers work. Water is often described as a universal solvent as many things will dissolve in it and thus, it can often be used to rinse stains out if they are caught fresh. Warm water is often more effective than cold because at higher temperatures, molecules tend to vibrate more vigorously, which means that they are not so “tightly bound” to each other and so easier to break the temporary attraction between the stain molecules and the material molecules.
A solution of water and detergent works on a stain by separating the stain molecules from the material and suspending it so that it is less able to re-attach. Sometimes, the stain molecules will not dissolve in water but will be soluble in other liquids, such as alcohol or special solvents, e.g., dry-cleaning fluid. In certain cases, the stain molecules need to be “chopped up” first by the process of oxidation, before water and detergent can separate them – this is what happens when enzyme or biological detergents are used on stain before a normal wash and a similar process is used by bleaches.
Stains from “permanent markers” tend to be impossible to remove because the black carbon contained in the ink soaks in to the pores of the fabric and binds tightly to the fibres; it is resistant to oxidation and not soluble in water and detergent, thus it is “permanent”.
Stains can be classified in several different ways but a popular one is to group them by the removal method. Thus you have protein stains, which require a soak in cold water and laundering with a biological detergent (e.g. blood, dairy products, mud, vomit, urine); tannin stains which must not be treated with soaps but with detergents instead (e.g. alcohol, coffee, fruit juice, berries, tomato juice); oil-based stains which require heavy duty detergent in hot water (e.g. oils, butter, mayonnaise, cosmetic creams) and dye stains which need detergent and possible bleach, if the fabric can stand it (e.g. inks & paints, grass, coloured children’s drinks, mustard). Lastly, many stains are combination stains which case you will need to use a 2-step treatment, such as first removing the oily/waxy portion and then removing the dye portion.