If I knew that my portrait was still at Antwerp, I would have it kept
back for the case to be opened, so that one could see that it had not
been hurt by so long a time spent in a case without being exposed to the
air, and that, as often happens to colours freshly put on, it has not
turned rather yellow, thereby losing all its first effect. The remedy,
if this has happened, is to expose it repeatedly to the sun, the rays of
which absorb the superfluity of oil which causes this change; and if at
any time it still turns brown, it must be exposed afresh to the sun.
Warmth is the only remedy for this serious mischief.
_Rubens._