When a soluble calcium salt, such as calcium chloride, is added to the filtered extract, solid calcium alginate is formed. If the calcium solution and filtered extract are mixed carefully, the calcium alginate can be formed as fibres - bad mixing gives a gelatinous solid. This fibrous material can be readily separated on a metal screen (sieve) and washed with water to remove excess calcium. It is then stirred in dilute acid and converted to alginic acid, which retains the fibrous characteristics of the calcium alginate. This form of alginic acid can be easily squeezed in a screw press. A screw press with a graduated-pitch screw is usually used; the squeezing action must be applied very gradually, otherwise the material will just move backwards and out of the press. The product from the screw press looks relatively solid but still contains only 20-25 percent alginic acid. However, it is dry enough to form a paste when sodium carbonate is mixed with it to convert it to sodium alginate. Sodium carbonate is added to the alginic acid in a suitable type of mixer until the required pH is reached, then the paste is extruded as pellets, dried and milled.