What happens if I wash my baker's couche?
Flax is a very problematic fabric to wash, and even more problematic to dry. You risk ending up with stiff, wrinkled, and/or shrunk fabric. 

I would never machine wash the flax couche. Actually, I wouldn't wash it after regular use, when it doesn't come into contact with anything but flour and lean dough. But if it accidentally got dirtied with something else, you are obviously better off washing it than throwing it away. 

In this case, I would hand-wash it in room-temperature water, neven in a washing machine (not even the wool cycle). Use wool detergent (no fabric softener), and don't crumple the fabric while washing. Instead, try to let it soak a bit (maybe 15-20 min) and then gently "shake off" the dirt under water. You can add a bit of food grade citric acid for the last rinse, because acid acts somewhat similar to fabric softener. 

When it is washed, don't wring it. Shake off as much water as you can, then lay it between two frotee towels. It should be spread out, not wrinkled or folded. Keep it at room temperature, not near sun, wind or a heater. Change the towels after 6 to 8 hours (the first time you may have to change them sooner). Repeat until it is only moist, not wet. Then spread it somewhere to dry without towels, again away from sun, wind, or excessive heat. Turn it every few hours. It will probably be stiffer than when you started with it, but I think it will be still usable.