There are two types of adsorption: physisorption and chemisorption. Physisorption, also called physical adsorption, is a process in which the adsorbent material and the adsorbate molecules (varnish precursors) do not form chemical bonds arising from a chemical reaction but are bonded by weak electrostatic forces arising from induced dipole moments such as van der Waals forces. The electronic structure of the adsorbate does not change upon adsorption. Because of its chemical structure, varnish molecules are believed to be attracted to the adsorbent through weak molecular forces such as hydrogen bonding.