Prepared atmospheres   
For heal treatment and other purposes atmosphere comprising of gases is prepared according to the requirement. American Gas Association (AGA) has classified atmospheres in 6 groups, on the basis of method of preparation

Exothermic base (AGA 100):
Prepared either by partial or by complete combustion of gaseous fuel with air. Water vapour may be removed to produce a desired dew point. The atmosphere is a mixture of .  In a lean exothermic atmosphere the ratio of  is greater than a rich exothermic one.

This atmosphere is prepared by burning a mixture of hydrocarbon fuel and air. The combustion products are passed to a condenser to remove water. The combustion products are further dried by using an absorbent such as activated alumina or activated silica. The dried atmosphere is then transferred to the furnace.

Typical applications include bright annealing of steel, copper, sintering of non ferrous metal powders, and iron powders

Prepared nitrogen base (AGA 200)

They are exothermic atmospheres and produced by combustion of a mixture of air and fuel gas.
Carbon dioxide and  are removed from products of combustion.
They are used to heat treat low-carbon, medium-carbon and high carbon steels.  atmosphere cannot be used for decarburization.

Endothermic base atmospheres (AGA 300).

Endothermic base atmospheres are prepared by using a lean mixture of hydrocarbon fuel with air, i. e. mass of air is less than stoichiometric (theoretical) amount required for complete combustion. The objective is to produce  and  .

As amount of air is less than stoichiometeric , a catalyst and extra amount heat are required to facilitate combustion. The endogas is cooled immediately to prevent the following reaction:

 

Typical applications include bright annealing of steel of any carbon content without decarburization or carburization, heat treatment of steel of any carbon content, bright copper brazing, and carrier gas for gas carburizing or carbonitriding.

Consider complete combustion of methane

 
For the ratio of   , the atmosphere consists of  and . However if the said ratio is less than 9.52 e.g. 1.88, then according to following, reaction,

 

the atmosphere comprises of  and . The volume of atmosphere would increase by 3 mole volumes. The atmosphere would comprise of  and .