Dealing with paintings in their frames poses a difficult set of tradeoffs. First remove all backing materials—paper, cardboard, Foam Core Board, or plastics.
Remove paper or cardboard backings from the reverse of the painting. They have done their job, and their continued presence will just complicate matters. This should be done as promptly as possible. If there are labels, save them, noting which painting they were from (i.e., document everything!). Don’t forget that labels are part of the painting’s history and establish its provenance.
If the frame is a strip frame, it is only holding moisture along the tacking margins and it should be removed eventually, but this is probably not the highest priority task. It is also likely that the nails holding the strip frame will rust, potentially causing stains in the tacking margins.
A shadow-box frame, without glazing, will provide structural support, so you may want to consider leaving it in place if it doesn’t restrict airflow to the reverse, and you can blot out any moisture between the edge of the painting and the frame.
If a frame is falling apart (like the gesso softening and smearing), it is best to remove the painting from the frame as quickly as possible.
If the frame is strong and has a traditional rabbet against which the painting is pressing, you will have to make a judgment call. The frame can hold water against the very edge of the surface and along the tacking margins, so for this reason it is best to remove the painting from the frame. However, if the canvas has shrunk and the stretcher is not sufficiently strong, leaving the painting in the frame will help restrain the stretcher and prevent torqueing.
Deciding which action to take should be based on the stretcher type and thickness, the construction of the corners, and how securely the painting is held into the frame. Raw canvas or exposed ground along the edges are more apt to be stained from contact with the wood of the frame, so in these cases it is probably best to remove the frame from the painting.
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