Take an old framed print off the wall and open it up. If there is a brown line just inside where the mount sat, that is mat burn.

What causes it

Wood-pulp mount board is acidic and contains lignin.

Held against paper for years in contact, it migrates and it stains.

Why it is always at the edge

Because that is where the board touches the work.

The line traces the aperture exactly, which is how you recognise it.

It cannot be undone

The stain is in the fibre of the sheet, not on it.

Conservators can reduce it. Nobody removes it.

The fix costs almost nothing

Alkaline-buffered, lignin-free board, and acid-free backing behind it.

It is a fraction of the moulding price and it is on every quote we send.