There have been many advices given here in the past as to an effective destruction of organic matter, such as the treatment with hydrogen peroxide solution or dilute mineral acids. Apart from the damage these chemicals can do to sensible minerals such as phosphates, they are quite ineffective in removal of coatings from a solid surface. Molds and lichens are often tightly attached to the surface; killing them does not mean that they will instantly come off. Strong chemicals do harm to living cells, but are not capable of signifcantly affecting the backbone which consists of polysaccharide derivatives that are pretty unreactive, except when treated with concentrated sulphuric acid, but this is certainly not an option here.
Understanding that removal of a coating from a solid surface is not always done by altering the coating, but often also requires to break the forces that make the coating or its decomposition products stick to the surface, is crucial in planning an effective cleaning strategy. For reasons I will not explain here, the surface of an ionic compound in contact with an aqueous solution acquires a moderate excess positive charge, a so-called surface potential. Likewise, the corresponding liquid surface at the contact acquires an excess negative charge, so there will be an electrostatic potential jump across the phase boundary. Hence, interactions at this boundary are mainly electrostatic, and the most simple way to influence them is by modifying the dielectric constant of the solution phase, according to Coulomb's law (this is often overlooked, because the dielectric constant is "hidden" in the proportionality factor). The dielectric constant of an aqeuous solution, in turn, sensitively depends on the ionic strength, i.e. the charges and molar concentrations of all ions present in the solution. In fact, material adsorbed to a solid surface can be removed effectively in some cases by applying a concentrated salt solution. However, this takes time and will also not work satisfactorily in the present case.
In general, it is best to apply chemicals that are designed to aid in desorption processes, and this is just what surfactants do. However, the surfactants have to be properly chosen. Good results are only obtained when their physico-chemical properties match up with the particular solid-dirt combination. Different cleaners will have different effects, and inappropriate surfactants may have no effect at all or even deteriorate the situation.
Dish washing liquids are suited to remove organic matter from minerals, because they contain combinations of anionic and nonionic surfactants. First, let the specimens soak overnight in water. This should always be done as the first step in a cleaning procedure, no matter what cleaning agents are applied. In this step, the pores become filled with water. If you don't, they will become filled with the cleaning solution, and you'll never succeed in fully removing the chemicals from the specimen later. Then, dilute the dish washing liquid 1:2 by volume with water and leave the specimens in this mixture for another day or so. Do not expect to see any improvement within minutes or a few hours. This process takes time, because it requires the surfactants to penetrate into the gaps between the coating and the surface. Take the specimens out, rinse them with water and brush them gently. This should bring a major part of the coatings off. Though, you'll likely have to repeat the procedure in order to fully get rid of them.
Another detergent to consider is a hair shampoo. Shampoos contain a combination of nonionic surfactants and cationic polymers. These have been designed to stick to the hair which has a negative surface charge. The effect experienced by the costumers is an apparent increase of the "hair volume". That's at least what the advertisements say, but it's actually just electrostatic repulsion. Apply the shampoo in the same way as the dish washing liquid. However, do not mix them: cationic polymers form very stable associates with anionic surfactants, making both ingredients ineffective.
Edited 3 time(s). Last edit at 05/04/2008 09:34AM by Peter Haas.
