Much of what I know has come directly to me through collecting art and framing my collection.
At first I was going to go chronologically through my collection. but I'm preparing one of my own paintings for a group show and am going to employ this very important preparation for my own work. So for me this is what's current.
I first learned about this valuable technique of backing my canvas's from my first real art instructor Mr Frank Liljegren formerly of New York, currently of St Mary's Ohio. I don't know if he still teaches there, but he is a master painter and a master teacher. Mr. Liljegren has a wealth of knowledge and never holds that knowledge back. His students walk away well prepared in the skills of the artists craft, as well as a passion for it. Franks love for art is infectious and his class's were electric with excitement for learning.
It was Frank that inspired me to move to New York City, he spoke so fondly of his time studying, and later teaching at the Art Students League of New York, that I just had to go.
Before leaving for New York I purchased one of Franks paintings, a gorgeous oil painting called The Coffee Pot. It was unframed at the time. When I went to pick up the painting from Franks studio, he had packed it very securely in a box for me and also handed me a large sheet of mat board with a hole cut into the center that was a 6.5"X4" rectangle. He also had a typed label with his name the name of the painting, medium and size adhered to the back of the mat board.
Frank advised me to bring this board with me to the framers and have them attach the board to the back of the frame. He was very precise in telling me to have it attached to the frame, not touching the back of the canvas, and that the hole/window in the board was so the canvas would have air circulation, it needed to be able to breath. This he said was very important to protect the painting from the risk of punctures or dents from the back. I have also learned that the canvas needs to have the circulation to protect against mildew forming due to humidity in the air.
The mat board is something that can be purchased from most art stores. The 100% acid free is more costly, but its what I would recommend for most applications. The less expensive partial acid free is only acid free on the back side, and it is only a veneer of acid free paper. That is the side that should always face the art work. Please do not use cardboard, only in a pinch to travel, or a temporary solution for added protection. I use cardboard for many tasks around the studio, but not for the framing of valuable works of art, and never for storage of works on paper.
For my group show, where we are displaying works unframed I use foam core for the back, it is a temporary but much needed added protection. This gives my painting a better chance of coming back to me unscathed. It will be shuffled about by many hands both skilled and unskilled in the handling of art. The foam core is not acid free but will work as a short term solution. I wont find the bulging impressions of other peoples smaller paintings having slipped behind and pressed against the back of my work during the installation or dismantling of the show. Foam Core boards can also be purchased in an acid free (archival) variation.
In the out of print book "Is Your Contemporary Painting More Temporary Than You Think?" The author Louis Pomerantz has a section on protecting a painting with backing, though he does not include a circulation window in his instructions, It is still a book I very highly recommend, it brims with information to artist and collector. You can still find this book online.
This last picture shows my temporary protective backing on my unframed canvas. A little crude, but it will do the trick, I also think the opening is too small for this size canvas. I used tape reinforced with staples to secure the board in place. A more permanent way to hold in place would be to use 1/8th inch brackets or some other hardware, rather than, or in addition to tape.
I hope this was helpful. Happy collecting!