Answer:
Because liquid solutions cannot penetrate the sheet, while gases can penetrate.
Explanation:
The paper is called acid when the internal bonding adding specific chemicals to the fibrous suspension in order to control the penetration of liquids in the sheet is carried out in an acid medium (pH = 4.0 to 5.0) with the addition of glue to the pitch-based, aluminum sulfate and, generally, kaolin as a mineral filler, just to name the main additives. Liquid solutions have difficulties in penetrating the leaves, for this reason, these solutions are not the best option to treat acid paper and it is preferable to use gases.
