Burning Surface Area
The burning rate - expressed either in grams/second or millimeters/second - will increase as the burning surface area increases. Small grains will burn faster than large grains due to their greater surface area per gram. Compositions loaded into a narrow tube should burn more slowly than the same material in a wide tube. The heat loss to the walls of the container is less significant for a wide-bore tube, relative to the heat retained by the composition. For each composition, and each loading pressure, there will be a minimum diameter capable of producing stable burning. This minimum diameter will decrease as the exothermicity of the composition increases.
A metal tube is particularly effective at removing heat from a burning composition, and propagation of burning down a narrow column can be difficult for all but the hottest of mixtures if metal is used for the container material. On the other hand, the use of a metal wire for the center of the popular wire "sparkler" retains the heat evolved by the barium nitrate /aluminum reaction and aids in propagating the burning down the length of thin pyrotechnic coating.
A mixture that burns well in a narrow tube may possibly reach an explosive velocity in a thicker column, so careful experiments should be done any time a diameter change is made. For narrow tubes, one must watch out for possible restriction of the tube by solid reaction products, thereby preventing the escape of gaseous products. An explosion may result if this occurs, especially for fast compositions.