We use the word “stain” to refer to anything that does not wash out of our garments or other things (carpets, driveways etc.) easily. To understand why, we need to zoom right down into the micro structure and see what’s actually happening.
Imagine that you took a packet of potato chips, crushed them between your hands, and then sprinkled them over a thick shag pile carpet. Vacuuming it up would not be easy, as the tiny little grains would fall into every nook and cranny in the carpet and get stuck as you tried to vacuum them out.
The little bits of chip would not actually be stuck to the carpet in the same way that it would be if it was glue, but simply physically entrapped. That is, if you got on your hands and knees with a pair of tweezers you would have no difficulty getting all the little bits out – it would just take an awfully long time.
Generally, this is what stains are. Generally, the substance in question is mostly physically entrapped and it just needs a bit of coaxing to remove. This can often be achieved by simply using a good detergent and knowing what cycle to set your machine to.
Other forms of stain are more strongly bound because of a chemical interaction of some sort between the stain and the molecular structure of the garment. In general terms the worst culprits here are natural fibres, particularly cotton and wool.
The reason in the case of cotton is that because it is a carbohydrate it is very oxygen rich, and oxygen is quite a reactive element. There are many substances which will form a bond with these oxygen groups and at a molecular level this can make the stains quite sticky, and limit the effectiveness of normal washing processes.
With wool, the problem is the sulphur which is is even more reactive than oxygen, and this has always been the downside of pure wool carpets.
Synthetic fibres, on the other hand, are polymer chains, and usually don’t contain as many reactive sites as natural fibres do.
Perhaps the worst type of stain is one that changes form after it gets onto the fabric – in other words, things that “set” like paints or resins. In cases like these, the degree of chemical bonding with the fabric is not the problem – the problem is the intimacy of the contact. That is, although the bonds between the paint and the surface are not particularly strong, the fact that there are so many of them, because the liquid ran into every nook and cranny before it set, means that the additive effect is quite substantial.
As it happens this is the principal behind many glues and the reason that they tell you to make sure that the surface is free of oil and water before the glues are applied.
Stains fall into several categories and we will now look at them individually and see what general characteristics they have.