Organic acids appear to form part of a non-enzymic process of cellulose decomposition that forms part of the wood decay process. Oxalic acid is produced in large quantities by fungi causing brown rot decay, and is produced also by many white rot causing fungi. The precise nature of the process of cellulose decomposition remains unclear but it is likely that the oxalic acid functions as a catalyst to enable a hydrolysis reaction to take place within the crystalline lattice of the cellulose component of the wood cell wall.

The oxalic acid seems to facilitate a hydrolysis of the glycosidic bonds of cellulose chains as a first stage in a multi-component cellulase system. This first stage enables the multi-component exo and endo-cellulase enzyme systems to gain access to the polymeric cellulose chains. The enzymic components of the cellulase complex are too large to be able to penetrate the capillary structure of the cell wall. Enzymes are protein molecules of molecular weight usually in excess of 10000. Bailley (1968) postulated that the destruction of cellulose by brown rot fungi was not entirely enzymatic. It had been demonstrated previously (Halliwell 1965) that in an in-vitro system, cotton cellulose could be completely solubilised using a system comprising ferrous salts and hydrogen peroxide in a similar way to that achieved by microorganisms. Such a non-enzymic initiation of decay comprising ferrous ions and hydrogen peroxide would certainly be capable of penetration of the capillary structure of the cellulose complex within the wood cell wall. Cowling and Brown (1969) suggested that this system might be involved in cellulose decomposition by brown rot-causing fungi. The feasibility of this was demonstrated by Koenigs (1972 a b 1978 1984 a b) Hydrogen peroxide is produced by brown rot-causing fungi when grown on wood. In addition he demonstrated that wood exposed to an in-vitro iron–hydrogen peroxide system displayed many of the characteristics of brown rotted wood, and that this wood was susceptible to degradation by purified cellulases.

The exact biochemistry of the process remains to be fully elucidated. However it is clear that brown rot and many white rot fungi produce organic acids, especially oxalic acid, and that this plays a part in the microbial decomposition of the cellulose cell wall. The cellulose component of wood cell walls represents around 30 % of the structure by weight of wood. The sensor enables the detection of oxalic acid produced by Serpula lacrimans in parts of a building where there is a sufficiency of free moisture.

Rypacek (1966), and Willeitner and Peek (1979) were able to detect fungal growth in wood before there was any visible evidence of decay, or any substantial (greater than 1% by weight) weight loss. It was further reported by Peek Willeitner and Harm (1980). They recorded a colour change reaction with 22 species of fungi responsible for brown rot (all the species under test) and 15 out of 25 species of fungi causing white rot.

The difficulty in using this principle in practice as an on-site survey procedure is, in part, because there is great variability in the natural acidity of wood in service. Wood also contains a large number of extraneous substances that interfere with both moisture absorption and desorption, and the colour change that enables visualisation. These difficulties have been overcome by using a pre-conditioned, vacuum-impregnated wood sensor manufactured from Gonystylus macrophyllum, A period of pre-conditioning is required to remove volatile and soluble extraneous extractives. After pre-conditioning and vacuum-impregnation with the colour reagent the absorption and desorption of moisture, and the colour change reaction can take place reliably and reproducibly in the presence of Serpula lacrimans.