'Conservation' comes from the Latin 'conservare' meaning: to preserve, to keep intact, to protect from decline.
With conservation, action is taken to stabilise an object that is deteriorating and to redress the damage it has sustained.
Damage occurs on two levels: a 'micro' and 'macro' level. On the 'micro' level, the highly organized fibre structures of paper, leather and parchment are exposed to chemical, biological and physical influences. These influences break down the fibre structure of cellulose in paper and the collagen in parchment and leather. Paper discolours, becomes brittle; leather and parchment loose their strength and may even crumble. On the 'macro' level we also contribute to a breakdown of our documents by inflicting 'wear and tear' on them or even worse by misusing them: drawings are torn and books fall apart.
By conserving an object damages are identified and treated.
On the micro level discoloured and weak paper can be washed to remove the discolouring chemical and biological breakdown products. Washing strengthens paper as it restores broken hydrogen bonds between the cellulose molecules.
On the macro level tears are mended with long fibred Japanese tissue papers and wheat starch paste. Lost areas can be filled in with contemporary or new matching paper or alternatively leafcasted whereby gaps are filled with paper pulp. Broken cords are mended with new hemp or flax fibres, boards are reattached and joints are repaired with insets of new leather or linen to make the book functional and accessible again.
The materials used to repair the damage in objects are of a proven stability themselves. They are either traditional and have stood the test of ages or are modern and have stood the test of accelerated ageing in a laboratory. The first are the likes of traditional starch pastes, animal glues like gelatine and vegetable tanned leathers; the second are the modern cellulose derivative and polyvinyl acetate glues and aluminium-tanned leathers.
Conserving an object is not making an object as 'new'. The work on the object will be kept as minimal as possible, to preserve its old appearance. Moreover the conservation is done with a high degree of reversibility, meaning that in principle it can be undone.
After (and before) conservation there is preservation (or preventive conservation), which is creating and maintaining the proper storage and handling conditions for the (conserved) object so that damage that happened to it in the past is less likely to happen in the future.
Irish Professional Conservators' and Restorers' Association (IPCRA)
Institute for the Conservation of Historic and Artistic Works (ICHAWI)
Trinity College Dublin Preservation and Conservation
British Library Preservation Advisory Centre
ICON - The Institute of Conservation