Change in temperature?
 

I recently delivered a portrait that was in perfect condition when I left it. A week later, the mother has called to say that the canvas now has "ripply waves" on the edges. She has it hanging over the fireplace and has stated that they have had many fires burning over the holidays. My studio stays pretty cool, if not cold at times. Is the reason for this stretching of the canvas due to the temperature difference? Is there anything I can do other than having the canvas restretched to fix this? I have never restretched a canvas, this makes me a little nervous. And last, I will do what I can to help her, but is this ultimately my responsibility?

I got into a lot of trouble once when I took a canvas completely off its stretchers...it wouldn't go back on and I had to repaint the darn thing!

Instead, on the back, try tapping in the wooden corner canvas pegs that (hopefully) came with your stretchers (or prestretched canvas). If you don't have these, any art store that sells stretchers will probably give you some.

If you're dealing with linen canvas, alas, it will always bag and sag more than cotton canvas. Changes in temperature and humidity will affect paintings in this manner...that is why you see climate control devices in most museums.

If you tighten the painting with pegs in the back, I don't see that there is much else you can do...your client might just have to live with it until the weather changes. Good luck!

Other possible solutions
 

I find that humidity more then temperature changes cause this but since winter is usually the dry time of year in areas that get cold maybe you are right that the temperature is causing it. You said that the painting is over a fireplace (not the best location since it will get smoke on it but I have 3 fireplaces and have a painting over each so I am also at fault at hanging paintings in this hash environment as well). I just mention that because the painting will need cleaning over time if a fire if burned in that fireplace a lot.

Try wetting the canvas on the back side then when it is wet, tighten it using the pegs. When the canvas dries it will be as tight as a drum.

Also I do not know how this will be in the long run but I have also applied a coat of acrylic gesso to the back of linen canvas and found that it also helps control this kind of problem. (helps keep moisture from getting in from the back.) If you do this the canvas will sag when the gesso is wet and tighten when it dries.

I would not resort to re-stretching completely unless none of this works. I would have the client take it to a good framing shop and have them do it if it needs to be re-stretched Also a acid free paper backing on the frame may help control moisture from the back.

I would recommend that you do NOT wet the back of the canvas if the canvas is oil primed. If it's acrylic primed it will be OK to wet it. I had the unfortunate experience of having a frame shop wet the back of one of my portraits, the paint fell off the front. Oil primed canvas is coated with water soluble rabbit skin glue before the white lead, or white paint is applied. If the rabbit skin glue is wet from the back it can have disastrous results. If you wet it at all do it very lightly with very little moisture.

I suspect if you try to expand your corners with wedges you'll risk causing problems with the frame I presume it's in? Not usually a lot of gap once a canvas has been framed. What do you do if you expand your corners and it won't go back in the frame?

Restretching doesn't have to be as daunting as you fear, although as Karin has experienced, don't remove the canvas completely! Try restretching one side at a time. Gently! Remove all tacks/staples on oposing sides and gently stretch as you re-tack. Then do the other two sides. Providing the sag isn't enormous to begin with, this should work.

Is the client prepared to relocate the painting for a period to see if it self-tightens? They may then consider another wall?

I agree with all above advice. Light spritz with spray bottle works well if its acrylic primed. Regarding your question about responsibility...I do have a clause in my contract about not being liable to damage, just in case..(although I do my darndest to make everything archival)

Hope it all works out!

Wetting lead primed canvas (thanks Steve)
 

Steve you are right I use acrylic gesso so I did not think of that. Glad you were out there, it could have been a mess if my advice had been followed on a lead primed canvas. Thanks