Paper makers use many different synthetic and natural chemical additives for a variety of different reasons during the wet formation process. They are used to influence the efficiency of the formation process or to impart specific sheet properties. They are usually added (with the exception of pigments) at a level of around 0-5% by weight of the other components of the furnish and, because of their relatively high cost, they often represent a significant proportion of the total raw material costs - particularly for recycled grades where the fibre costs may be very low. When it is necessary to modify bulk sheet properties, as in the control of aqueous fluid penetration (internal sizing), the chemicals have to be added to the wet fibre suspension so that they become well distributed throughout the z-direction of the sheet. Chemicals which are added as a surface treatment to the dry sheet are usually only able to influence surface properties.