MadSci Network: Biochemistry |
Dear Gabriella, Paper deteriorates over long periods from the effect of a number of factors - not least, the fact that paper-making involves the use of chemicals which, remaining in the fibres themselves, slowly cause the paper to break down. These factors are: Mechanical (opening and closing a book) Chemical - see above Heat and moisture Light - this causes lignin to turn brown (obvious in an hour with newsprint) Air pollutants - particularly soot - common in churches etc, where centuries of candle-burning has resulted in many murals and old Bibles becoming very dark. Microbes (e.g. fungi) can also damage documents through: 1. rotting the cellulose fibres making in weaken 2. digesting the 'size' or glue that sticks the fibres together and the filler that is used to make the surface smooth and aborb the ink. This includes xanthan (gum) and starch (filler) 3. producing pigments which discolour or otherwise mark the document (known as 'foxing') 4. digesting the inks/dyes/pigments that are used to create the text of the document. In each case, the digestive process is a hydrolysis reaction in which extra-cellular enzymes add water to the polymer, reducing it to simpler molecules, which can then be absorbed and used by the fungus. Examples include: Starch + n(water) > n(maltose) - using amylase (itself a mixture of at least two enzymes) Cellulose + n(water) > n(beta-glucose) (using cellulase) Since fungi, like all living things, need water, if the relative humidity is over 70%, fungal growth is inevitable; thus, the use of silica gel in document drawers is common-place. Many of the chemicals in paper are HYDROSCOPIC and so absorb water from the air, thus helping the fungi! Many documents have been damaged or destroyed by burst pipes or flooding - particularly those in private houses. Fungal spores can survive high temperatures, so the use of heat to kill the spores is NOT a good idea, though keeping documents in cool conditions will slow the action of fungi right down! Temperatures or 80-100 degrees Celsius CAN be used to stop the growth of any existing fungal hyphae - though this must be done in an oven and for short periods of time only! Most disinfectants do more harm to the paper than the fungi do, and, being liquids, are not useable, though the use of certain gases IS possible (e.g ethylene oxide). Filters and sealed containers can keep fungal spores out, thus slowing the onset of decomposition. Aspergillus fumigatus and A. flavus (italics!) can damage documents AND infect humans too, so there is a potential health hazard in poring over mouldy old tomes! Cellulase is INHIBITED by Vit K, so this has been used as an enzyme inhibitor to reduce damage; U-V light also kills fungi without seriously damaging the paper itself. The use of 60% CO" or 100% N2 atmospheres also slow or stop fungal growth - and can be used in display cabinets. These days, most critical documents have been photgraphed or scanned, so that students can examine them on micrfiche or on computer, without risk of damage (or theft) of the original. Insects (and primative 'silverfish'), together with rodents can also do considerable damage to documents, though whether they are better informed having digested the contents of the documents is not recorded in the literature! These two web-sites give even more detail: http://preserve.harvard.edu/resources/postcards.html and, http://www.uni-muenster.de/Forum-estandserhaltung/grundlagen/nitterus.shtml. A simple school experiment is to place pieces of filter-paper in various environments inside a petri dish, and record the rate of fungal growth that can be seen over the following weeks. You could try damp soil, damp peat, damp sub-soil, damp leaf litter, watering with tap, bottled, distilled or pond water and keeping the samples at room temperature, in a fridge or in an incubator set to 30C; the controls would be DRY samples kept under the same conditions. Hope that helps! ian
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