Crystal lattice breakdown refers to the removal of ions from the minerals of which the stone is composed. Removal of ions requires that the chemical bonds between molecules are broken. This requires an energy input, the strength of which will vary depending on the strength of the bonds and the presence of catalysts. Crystal lattice breakdown requires the presence of a substance capable of reaction, as well as a mineral capable of reacting with it. Any chemical reaction involved is an exchange rather than a straightforward removal of ions. There are two basic exchanges, exchange of a proton and exchange of an electron. The former can involve acid solutions, while the latter is the redox (reduction and oxidation) reaction. For removal, it is essential that the reactants can react, that there is sufficient energy to overcome the bonds within the mineral and that the products of reaction are removed from the surface where the reaction occurred. Delays in removal of reaction products means that a layer of unreactive material can build up on a surface, restricting further reactions and so slowing weathering. Where products are removed, such as in flowing water, the surface reaction may be reduced in its effectiveness as the two reactants may not be in contact long enough to permit the reaction to occur.