Scrims are made of Lignin.
They are graded as someone has mentioned, grade 1 are the better ones.
For those who have trouble breaking them in, try boiling them or as I used to do put them in the washing machine, on a hot wash (no soap/chemicals) when new, then use them to wipe down frames / ledges, use them to mop up spills etc, wash them out on a rinse cycle in your washing machine, they soon break in.
You can buy pre bleached scrim, which are ready to use but are not as good and dont last as long