Impression material is defined as any substance or combination of substance used / for making and impression / negative reproduction. Impression is defined as a negative likeness or copy in reverse of the surface of an object, an imprint of the teeth and adjacent structures for use in dentistry. Master impression is defined as a negative likeness made for the purpose of fabricating prosthesis. Impression technique s a method / manner used in making a negative likeness. 

Desirable properties of an impression material are

• A pleasant odour, taste and acceptable colour 
• Absence of toxic or irritant constituents 
• Adequate shelf life for requirements of storage and distribution 
• Economically commensurate with the results obtained 
• Easy to use with the minimum equipment available 
• Setting characteristics that meet clinical requirements 
• Satisfactory consistency and texture 
• Readily wets oral tissues 
• Elastic properties that allow easy removal of the set material from the mouth and good elastic recovery 
• Adequate strength to avoid breaking or tearing upon removal from the mouth 
• Dimensional stability over temperature and humidity ranges normally found in clinical and laboratory procedures for a period long enough to permit the production of a cast or die 
• Compatibility with cast and die materials 
• Accuracy in clinical use 
• Readily disinfected without loss of accuracy 
• No release of gas or other by-products during the setting of the impression or cast and die materials 

Classification of impression materials  

Based on mode of setting and elasticity 

Rigid

• Reversible / thermoplastic – Eg. Impression compound 
• Irreversible / thermosetting – Eg. ZOE impression paste, Impression plaster 
• Reversible - Eg. Agar 
• Irreversible – Eg. Alginate 

Elastic 

• Hydrocolloid 
• Elastomeric impression material – Eg. Polysulphides, Polyether, Addition silicones, Condensation silicones 

Based in type of impression and area of use 

Dentulous – 

Primary – Eg. Alginate 

Secondary – Eg.. Elastomers, Agar 

Edentulous 

Primary – Eg. Impression compound, Impression plaster, Alginate 

Secondary – Eg. ZOE impression paste, Elastomers for CD 

Based on their use in dentistry 

• Edentulous – 

Eg. Impression compound, ZOE impression paste, Alginate, Elastomers 

• Dentulous –  

Eg. Agar, Alginate, Elastomers 

Based on the amount of pressure applied 

• Mucocompressive -  Eg. Impresion compound 
• Mucostatic – Eg. Impression plaster 

Based on manipulation 

• Hand mixing – Kneading, Circular motion, Vigorous mixing 
• Mechanical mixing 

Based on the tray for impression 

• Stock tray 
• Special tray 

Article by Dr. Siri P. B.