The sol gel method  is distinguished from other routes of material preparation from solutions or melts such as precipitation and crystallization by  two main characteristics:

1. Formation of clear colloidal solution due to primary condensation of dissolved molecular precursors.
   
   2. These colloidal particles merge during subsequent gelation stage into polymeric chain by chemical bonding between local reactive groups at their surface.

Both stages are controlled by condensation chemistry that can include as a first step, hydrolysis of hydrated metal ions or metal alkoxides molecules. The condensation chemistry in this case is based on olation/oxolation reactions between hydroxylated species. Olation is a condensation process in which a hydroxyl bridge “–OH--” bridge is formed between two metal centers.

The oxolation is a condensation reaction in which an oxo bridge “-O-” is formed between two metal centers. The steps in sol gel processing are shown in Fig. 2 and discussed below.

Activation and  polycondensation:  Metal alkoxides are used as precursors in sol-gel operation. Metal alkoxides are most extensively used as these are commercially available in high purity and their solution chemistry is well documented. For preparation of alumina and zirconia aluminium propoxide and zirconium propoxide are used respectively as precursors.

The metal alkoxides are hydrolysed in alcohol solution containing a controlled amount of water. The sol gel chemistry can be represented by following two reactions: 

Where M= metal ; X = H or  R (alkyl group)

This is a very simplified representation without giving details of the intermediate or end products.  However this gives an idea of formation of three dimensional gel network coming from the condensation of partially hydrolyzed species.