Must a turbid liquid. In order to obtain clean, clear wine the must has to be filtered. This also takes out the deposit formed after clarification through addition of finings. Filters can act by sieving (holding the substances which are larger in size than the pores of the filter), adsorption (holds the substances in the wine which have an electric charge opposite to that of the filter), depth (particles smaller than the filter mesh are stopped, as they remain trapped among the tangle of fibres which make up the filter).
To increase filtration capacity, obtain constant flow, encourage deposit removal and reactivation of the filtration accessories and adjuvants are required; these are substances which are chemically inert (therefore they do not react with the wine), such as filtrin, diatomaceous earth, perlite, cotton and cellulose.
As well as different technologies there are also different filtration methods; the rough or "sfecciante" method is used on young wines which have a lot of substances which make them cloudy. The "alluvonaggio" method is suitable for clearing large amounts of wine. 'Brightening' makes the wine very clear by passing to through layers of cellulose cardboard. This is usually done with wines which have already been depth filtered using diatomaceous earth. Lastly comes the sterile or micropore filtration to eliminate all the micro-organisms from the wine. They are held back because the pores are extremely small; less than one micron in diameter. Recent developments include ultrafiltration and tangential filtration. These techniques filter larger particles which often cause faults in a wine.
It is said that filtration 'stresses' the wine which is tired and worn out immediately after the operation due to loss of carbon dioxide and air intake. This is, however, a temporary fault which disappears in a day or two.