i use alot of chalk for marking quilts, even for piecing/applique--- i use alot of chalk.
i've never used ones that disappeared with an iron- i've always avoided pressing anything marked- fearing i would (set it)
i've never had a problem with chalk not washing out...different colors, white, they have all washed out of finished projects. i have looked at the 'chalk lines' that are being sold in quilt shops & thought about them- but found a carpenter's chalk line costs alot less- and has a whole lot more chalk in it- i've used one of those a few times to get a straight line around the perimeter of a finished quilt i was trimming before adding binding- it worked great...was alot easier than crawling along on my knees with my 24" ruler and a pencil.