Various types of filtering crucibles can be used, provided that the filtration is reasonably fast and all of the lignin is retained on the
filter, resulting in a clear filtrate. Glass filtering crucibles with a sintered glass disc of a fine (F), or medium (M) porosity can be
used on wood and on most of the pulps. Lignin in low-yield sulfite pulps forms a fine dispersion, which often clogs the pores of
the sintered glass discs and slows the filtration. A disc of a glass fiber paper, fitted in the crucible, facilitates the filtration.
Alundum or porous porcelain crucibles, with a mat of glass fibers, may also be used.