First, I would never wash an antique quilt in a washing machine, at least not one which has an agitator as the quilt will get wrapped around the agitator and will more than likely be ruined.
I would also never wash a quilt containing wool or silk, nor would I dry clean an antique quilt. Wool and silk are especially fragile. Dry cleaners use strong solutions and are more likely to hurt a quilt rather than save it.
Also, there must be no damaged areas–areas where you can see the batting poking out, rips, or places where the dyes (or other things) have eaten through the fabric. These are too fragile to withstand cleaning.
Most quilts in good condition (without damage mentioned above),can be cleaned first by vacuuming. You need to get a screen, fiberglass window screening available by the yard at your hardware store works well. Tape or sew binding on the edges to prevent the sharp ends from pulling threads or snagging the quilt.
Prepare a large table, or the floor by laying down a white sheet. Place the quilt on top, then the screen on top of that. Take a vacuum which is set at the softest suction possible by adjusting the air flow and hold the vacuum OVER the quilt (not directly on it). The vacuum will suck up dirt without pulling up the fibers. The screen helps prevent any damage to the quilt and holds all the fibers down. Vacuum the entire top of the quilt, then turn it over and vacuum the back side. This often is enough.
Some cotton quilts may be cleaned in the following way, but it is important to test the quilt to make sure that the dyes are stable.
Test each of different fabrics as follows: take a try Q tip and rub it over each of the fabrics. IF the color transfers, then it is likely it will run. If it passes this test, then dampen the q-tips and rub it on each of the fabrics. If it comes off onto the q-tip, then it will run.
If you have come this far without it running, then for the discolored spots, you can mix up a solution of non-chlorine bleach, specifically, I would use oh bother…Oxydol? It is in a blue container…and it is a really good addition to your washing…darling daughter is singing in my ear and I am having a hard time remembering. I’ll check in the basement later. Spot treat the areas using a spoon or a piece of muslin to tamp it on.
Draw some cold or at most tepid water into the bath tub…put in approx. 2 T of Orvus paste (you can get this at quilt shops, from Clotilde, or from some Livestock supply places (cheapest there….it was originally designed for washing Livestock–it is straight sodium Lauryl Sulfate and is gentlest on your stuff) into about 4 – 5″ of water. Buy one of the Woolite or CArbona “dye mops” to absorb dye which leaches in to the water…as a precaution. Lay the screening on top of the water and place quilt on top. Put dye mop on top of that. Pat the water through the quilt, not wringing. continue patting. Do not let the quilt soak as if the dyes go into the water and resettled, that is what contributes to the running of the dyes.
Drain off water, and run cold water through until the water comes off clear and no suds.
Press water out of quilt. Use towels to blot more water out. Lift the quilt on the screen (textiles are at their weakest when wet) and lay the quilt flat to dry…either on top of a clean sheet laid on the floor or outside. If you are putting the quilt outside to dry, cover the top with another sheet so that nothing falls on the quilt. Inside, put a fan on the quilt. After the top seems dry, turn it over and let the back side dry.