filtration
/ fil-TRAY-shun /
Think of pouring tea through a strainer: the liquid passes, the leaves stay behind. Filtration is exactly that — using a porous barrier to separate a solid from the liquid it is sitting in.
Filtration passes a suspension through a medium with tiny holes, such as filter paper or a sintered glass disc, that lets the liquid (the filtrate) through but holds back the solid. In gravimetry it is how you collect the precipitate so it can be washed, dried or ignited, and finally weighed.
It matters because every grain that slips through the filter, or every wash that redissolves the solid, is mass lost from your answer. Choosing the right pore size and medium — fine enough to catch the crystals, robust enough to survive ignition — is a quiet but decisive part of a good determination.
After barium sulfate has settled, the mixture is poured through ashless filter paper in a funnel; the clear filtrate runs out below while the white solid stays on the paper, ready to be ignited paper and all.
A porous barrier keeps the solid and lets the liquid pass.
Match the filter to the next step. Precipitates to be ignited go on ashless paper or a porcelain crucible that survives the furnace; precipitates only to be dried are better collected in a glass or Gooch crucible.