Foam extinguisher is filled with aqueous solutions of acidic (aluminum sulfate) and alkaline (sodium bicarbonate) chemicals. When in use, the mixing of the two solutions causes a chemical reaction to produce CO2 foam.

Al2(SO4)3+6 nah co 3 = = 3 na 2 so 4+2Al(OH)3↓+6 CO2↓

The residual Na2SO4 and 2Al(OH)3 during fire extinguishing will damage books.

Dry ice fire extinguishers should be used in library fires. Dry ice is low-temperature CO2, which will naturally volatilize during fire extinguishing and will not cause damage to books.