“If a piece of wet skin, tanned or untanned, is heated slowly it will reach a temperature at which it shrinks dramatically to about one third of its original area”
Kite, Marion and Ray Thomson. Conservation of Leather and Related Materials, Butterworth-Heinemann, 2006
Treatment of parchment damaged from excessive heat.
After photographing the document, I lightly surface cleaned it with grated vinyl eraser and began a long process of slowly humidifying and trying to pull the vellum back into alignment. Originally, the gap in the middle of this document was almost 4cm. The treatment consisted of humidifying, pinning out, humidifying until I felt the vellum would not move any more. Perhaps some of the areas around the shrunken bits were close but not over the shrinkage temperature. Three times, the vellum was humidified, stretched, dried and pinned out.
After repairing the losses with toned tissue, I constructed a thread mount. The owner wanted to put this document on more of less permanent display, and given the large area of losses, I wanted a mounting system that would keep the two main halves as planar as possible. The beauty of a thread mount is that if the humidity increases, the vellum relaxes and the thread swells slightly, shortening in length and taking up the slack of the relaxed vellum. If the humidity is lowered, the vellum shrinks and the thread shrinks, causing the twist to loosen and lengthen. If the thread is properly chosen and prepared, this system of mounting can accommodate a range of changes in humidity.
The next step was to mat the document, hiding the threads, then it was framed, care being taken to add spacers so the threads were allowed to move.
Peachey Conservation 
Very interesting project, a great repair to an almost lost document.
I have mounted Vellum before but not like your method.
Looking at your photo, I presume you have mounted the Vellum by threading round the edges of a Museum rag board with a continuous length of thread to maintain an even tension crossing on the back (or a short diagonal) to the next position?
Or are they individual threads and tied?
What type of thread did you use?
Do you have a close up of your stitching method?
I normally use a cotton thread weaker than that, which I am mounting.
Kind Regards
Jan Stanlick GCF
You have asked a number of very complex questions that vary with each item to be mounted. Different documents need different mounting methods. Given your questions, it may be best for you to consult a local conservator who is familiar with the wide variety of mounting options available. Failing that, I suggest you consult the literature about thread mounting starting with Pickwoads article in the Paper Conservator:
http://www.tandfonline.com/doi/abs/10.1080/03094227.1992.9638579#preview
There may be workshops in England that cover this technique?
Dear Jeff – would you mind if I talked about this page and shared your images – with full credit at a talk i am doing in house for some colleagues in Tunisia. The event is a teaching event with no fees charged to attendees and I would include your URL and use quotation marks and cite you,
Sure, feel free to use it! Thanks for asking!!