Advantec Quantitative Ashless Filter Papers are made from the highest quality alpha cotton cellulose. One critical step in the manufacturing process of these filter papers is acid washing. All grades are double acid washed in hydrochloric then hydrofluoric acid to reduce levels of SiO2, CaO, and Fe2O3. Following the acid wash, all papers are rinsed in ultrapure water to neutralize. Grade No. 4A Hardened is further treated with nitric acid. Following the acid wash, all papers are rinsed in ultrapure water to neutralize. Ash Content: 0.01% (except Grade No. 4A is 0.025%).