There are several steps in using powder paints/earth pigments to paint rawhide:
Understanding Why Rawhide Warps
Bottom Line: Rawhide will always be subject to warping due to humidity. Also, since rawhide is not uniformly thick it will expand and shrink accordingly. Additionally, if it is painted, the rawhide will absorb some moisture, causing it to expand in the painted areas but not in the unpainted areas.
For rawhide that is not permanently stretched in a frame, etc., there is no ready solution to be able to get it perfectly flat and for it to stay that way. It will always be warping whenever the humidity changes. (Even bringing it out of the house for several hours, then returning it can subject it to humidity changes.)
The only good partial solution to keep rawhide fairly flat is to physically wrestle with it and try to bend it over the knee, etc., in the areas that are warped the worst in an effort to get it somewhat flat.
Keeping it Flat: The “Old Way” to Make a Shield:
If really FLAT rawhide that won’t change shape is desired, then a fresh green hide is required. (The hip section of a buffalo or cow is desired – NOT the hump, as has been reported erroneously in the literature.) This should be a section about 3 times the size of the desired finished shield, and then it must be fleshed and de-haired.
Making rawhide that stays flat: FIG 1
Dig a pit about 15″-18″ deep with a diameter a little larger than the size of the finished shield.
Heat enough rocks to half fill the pit. This will take 3-4 hours.
After the rocks are heated, place them in the pit, and then the rawhide is staked all around the edge of the hole except for about a 12″ gap. See Figure 1.
The un-staked edge is lifted and water is thrown in on the hot rocks. See Figure 2.
Then the rest of the hide is staked down.
The rawhide will immediately start to visibly shrink, and the crafter must continually rub it with a wet rag to keep it from getting hot spots from uneven heat distribution.
Every so often, the hide has to be re-staked as the edges of the hide move closer in to the edge of the pit, and the hide will be shrinking and start to pull the pegs out.
Making rawhide that stays flat: FIG 2
When the hide is re-staked, more water must be sprinkled on the rocks and sometimes new hot rocks are put in, as well.
This process goes on all afternoon, but, at the end of the day, the hide has shrunk to about 1/3 its original diameter, is about 3x as thick, and unlikely to be noticeably affected by humidity.
Of course, when the shrinking is done, the peg holes should be cut off around the edges to make a nice, neat shield.
This is normally an all-day process and must be done if one wishes to make a completely authentic shield that will retain its shape regardless of almost any conditions.