The main cause of acid damage to artwork is by acid leaching out of the mountboard and into the artwork. Mountboards made from wood pulp can contain lignin. Lignin occurs naturally in trees providing structural support to the plant, unfortunately for artwork, it becomes acidic when it breaks down, this is why newspaper turns yellow over time. The two images below show the effects of acid damage on artwork:
The acid leaches out from the bevel or cut edge of the window mount and attacks the surrounding cellulose causing paper made from wood pulp to discolour and weaken.
To prevent this from happening, conservation board or museum board must be used. Conservation board is less expensive than museum board, it is made from wood pulp but has had the lignin almost entirely removed. In addition to this, the mountboard is usually buffered by the addition of calcium carbonate (chalk). This increases the alkalinity of the board and helps to counteract acid attack. Museum board on the other hand is not made from wood pulp at all but cotton. It is sometimes termed “rag board”. It is the most stable of all the mountboards and offers another level of protection for artwork.