Hi ChrisW
This is going to be a somewhat long explanation, but an understanding of the dynamics of the problem is necessary. I'm not going to edit this.
I suggest that you get a coffee.
The sheepskin is very hygroscopic i.e. it absorbs water - from its environment. (It also releases water, if its environment is dry.)
As it absorbs moisture it changes shape; the skin's cells expand one at a time, increasing their volumes, one at a time, until they either absorb what moisture is available or until they have reached their capacity.
As the cells fill out, they increasing the volume of space that they occupy - but - they do not do this at the same rate nor with equal capacities - which causes differential expansion to occur.
Natural skins (as do some plastics) have a "shape-memory". This is a naturally-occurring phenomenon in all organic matter. As the skin expands it is simultaneously subject to its "shape-memory" and to differential expansion, which results in the very exaggerated cockling that you see.
The procedure reverses if the environment is dry: the skin gives up its moisture to the environment.
When rolled up and stored, the skin's moisture content was in equilibrium with that of its storage environment in the customer's house. Its shape was determined by the walls of the tube in which it was rolled.
When it was removed from its storage tube in your shop, it undoubtedly stayed tube-shaped and did not just flop out flat on the table, did it? When opened flat, it wanted to roll up again, right? - shape-memory.
You unrolled it and sandwiched it, "for a few days", between weighted matboard, to flatten it. While sandwiched, the skin acclimatized to the new moisture levels in its new environment - between matboard which was stored in your shop (same moisture level as the rest of the environment in your shop) and in the RH present in your shop. Its new shape was determined by the weighted matboard i.e. flat. This, I assume, is where your customer saw it in its "flat" condition.
It was conservation framed under a window mat, then taken by the customer, to a new environment - where it promptly changed its shape. Its new environment also had a new RH - but, with nothing to constrain it or force it to keep its flat shape - the window in the mat allowed it to move unrestrained, and with the help of some shape-memory, it ended up looking "...like a 3D relief map."
It is my understanding that there is not a whole lot that can be done to change this phenomenon, if skins are to be treated in a conservation manner. Natural skins will behave, well, naturally, and unfortunately, if you will excuse a really bad pun, it's the nature of the beast.
In an ideal world, its cockling may be minimized (but not completely controlled) by making an effort to keep the scope, frequency, and the rate of change of its immediate environment, particularly that of RH, reduced.
Scope and frequency - Ideally, the skin should be flattened out and framed in its destination environment, or in an environment that closely duplicates it, particularly in regards RH. Since it will not be moving through differing environments, the frequency of environmental change will have been automatically reduced to zero.
Rate - Since an environmental constant, once at the frame's destination, cannot be guaranteed, steps should be considered to reduce the rate at which potential changes to the skin's environment inside the frame package may occur.
Conservation frame the piece as you normally would: glass, framespace, rag matboards, skin, rag mountboard, rag filler - stop! ...add more rag fillerboards here - it was suggested that three or four boards may be adequate; in this case, more is better - seal the edges of the mat package with conservation-grade tape, fit the package into the moulding, and then install an impermeable backer (or Tyvek, correctly oriented in regards its face) instead of dustpaper.
The goal is to reduce the rate at which changes to the environment inside of the mat package may occur. The tape seal's primary function is to prevent exchange of atmosphere - remember, this is supposed to be done in conditions that duplicate its final destination - if it was assembled initially in a relatively dry atmosphere, so much the better.
The extra rag fillerboards' primary function is to add physical mass that will buffer radical changes in temperature, mass of airflow, and airflow rate inside the package.. Everything is forced to slow down so that the skin can s-l-o-w-l-y and more gently acclimatize.
This is not a guaranteed solution. Some trial-and-error may be required. Be patient - skins are a tough problem. (Was that another bad pun?)
I hope that this will be helpful to you and your customer.
ABFF
[This message has been edited by Orton (edited 05-19-99).]