once the item is washed the marks are gone....but till then you can iron and then freeze then iron etc. many times.
we stopped 'testing' at 25 times on a fat quarter...then washed it and it marks did not re-appear when frozen....do not know if chemical is actually truly gone or just not visible.....
as for the students who buy used but unmarked textbooks and then put them in the freezer...hope the previous owner was an 'A' student......
In middle school the blank piece of paper that is passed in class...becomes a note when wrapped around a cold juice or soda can at recess.........tech savy kids today