The desirability of increasing the bulk or volume of tobacco has long been recognized.  Among the various reasons for desiring such increase in bulk or volume are compensation for the weight loss caused by the curing process.  Increase in filling power permits the use of smaller amounts of tobacco in the production of firm cigarette rods or the like and results in a lower delivery of tar and nicotine than a comparable product made of unexpanded tobacco. 
Many different methods have been suggested in the art for puffing or expanding the volume of tobacco.  For example, in U.S. Pat.  No.  1,789,435 to Hawkins, tobacco is expanded by subjecting tobacco to a gas under pressure, which causes the gas to penetrate the constituents of the tobacco.  Thereafter, the pressure is suddenly released whereupon the gas trapped in the tobacco expands.  Among the gases mentioned as usable in the process are air, carbon dioxide and steam.  The gas may be heated to facilitate the process. 
According to the method of British Pat.  No.  1,331,640, tobacco is impregnated with a compound capable of liberating a gas under conditions which do not impair the quality of tobacco.  Non-gaseous chemical compounds which liberate gases, such as carbon dioxide, nitrogen, oxygen and ammonia, upon thermal decomposition may be employed.  Preferred compounds are those which decompose at relatively low temperature including ammonium carbonates, ammonium carbamates, organic dicarboxylic acids and peroxides.  In one embodiment, tobacco is impregnated with an aqueous solution of ammonium carbonate and is then heated with a stream of live steam at 120.degree.  C. to cause it to expand. 
Tobacco expansion processes for enhancing the utility of tobacco stems for use in tobacco products are described in U.S. Pat.  Nos.  3,409,022, 3,409,023, 3,409,027 and 3,409,028 all to Burde.  According to the processes described, tobacco stems are expanded by means of radiant heat or microwave energy.  A method of preparing reconstituted tobacco sheets from puffed tobacco are also described.  A further enhancement of the puffed product obtained from tobacco stems is described in U.S. Pat.  No.  3,425,425 to Hind.  In this patent, tobacco stems are treated with a solution of water-soluble carbohydrate prior to expansion. 
In U.S. Pat.  No.  3,771,533 to Armstrong et al, assigned to the same assignee as the present invention, the various disadvantages of the above-described and other prior art processes of tobacco expansion are noted.  For example, some of the methods involve only moderate expansion, are not effective on tobacco leaf as well as tobacco stem, require elaborate and expensive equipment and/or involve introduction of foreign materials into the tobacco. 
According to the expansion method of the Armstrong et al patent, tobacco is treated with liquid or gaseous ammonia or with ammonium hydroxide or a combination of ammonia and carbon dioxide followed by heating to temperatures of from 250.degree.  F. to 700.degree.  F. for a time sufficient to puff the tobacco.  Carbon dioxide may be added before, during or after the ammonia is introduced.  The carbon dioxide may be introduced as a gas, or in powdered form, or in combination with the ammonia in the form of ammonium carbonate or bicarbonate which may be applied directly or formed in situ. 
Other methods have also been proposed to overcome some of the difficulties of the presently known techniques.  For example, in copending application Ser.  No.  912,029, filed June 2, 1978 to Lendvay et al and assigned to the same assignee as the present application, a continuous process for impregnation and expansion of tobacco using ammonium hydroxide and solid carbon dioxide or using aqueous ammonium carbamate as an impregnant is provided.  That process avoids certain difficulties of the known ammonia carbon dioxide process, for example, by avoiding build-up of ammonium salts. 
The present invention provides an improved method for heating tobacco which has been impregnated with ammonium carbonate and similar materials over both the Armstrong et al and the Lendvay et al heating methods, as well as over other prior art techniques