All oil paintings are fixed to stretchers, so this is something else. During the conservation process, a painting may be removed from its faulty old stretcher, laid down on a new canvas, which is itself fixed to a new stretcher of conservation quality. Sometimes the old stretcher is thinned down and retained, no longer really serving a structural purpose. Sometimes it is replaced when the painting is relined. I think this is what is under discussion here.

See the information given here: https://books.google.co.uk/books?id=My0sBgAAQBAJ&pg=PA159&lp...

Here is a discussion in French of what a bâti d'extension is: http://www.chassitech.com/bal2/

And here is a description in English of how these replacement stretchers are used in association with, or to replace an original stretcher:

B. ATTACHMENT OF AN ORIGINAL STRETCHER TO A REPLACEMENT SUPPORT




1. Purpose

Retaining the original stretcher, or parts of it, after the completion of treatment when the original stretcher is no longer part of the painting's support system, is an option. This technique is seen on occasion but has never become standard practice. The primary purpose of keeping the original stretcher, or parts of it, with the painting is to preserve the information written on the stretcher or to retain the original stretcher in order to document the original assemblage of materials that comprised the painting as a whole artifact. In other cases, the original stretcher performs a limited but important function as part of a new support system.

2. Materials, Methods, and Variations



a) General DescriptionThe original stretcher is removed during treatment and is later attached to a replacement stretcher. The original stretcher or the pieces serve no purpose in the new support system other than to preserve and communicate information that was found on or embodied by the original stretcher. The advantage to this extra step of the treatment is that the stretcher or strainer is retained (not just a drawing or photograph) and kept with the painting for future reference. If the stretcher is saved but separated from the painting, it can easily be lost; even more easily, the significance of the stretcher is lost and the stretcher is then thrown away. The disadvantage of attaching the old stretcher to the new may be the extra bulk added to the support of the painting and/or the extra step of treatment.The following are examples of this:(1) THINNING THE ENTIRE STRETCHERFrog Pond, Gloucester by John Sloan, American, 20th centuryDelaware Art MuseumThe original stretcher was thinned to 3/16" thick and 1 5/16" wide and screwed to a replacement stretcher at the time of a wax lining treatment (fig. 87a). This treatment occured before the 1996 acquisition of the painting by the Delaware Art Museum. In the John Sloan Trust collection of paintings at the Delaware Art Museum, there are several other examples of this same treatment. The original stretchers are saved for inscriptions by the artist indicating title, date, initials, color notations, and sometimes varnish information. The trimmed original stretcher acts as a collar around the backing board (fig. 87b). In most cases, the trimmed stretcher is screwed to the replacement stretcher; in one case, brass escutcheon pins are used. The trimming of the original stretcher has the advantage of eliminating the bulk associated with retaining the stretcher in this type of procedure.

http://www.conservation-wiki.com/wiki/PSG_Stretchers_and_Str...