Extension posts are metal screw posts that hold the album together through holes in the front and back cover. This makes the album expandable and allows page protectors and addtional pages to be easily fitted. It also means that you can scrap in no particular order and reorder pages whenever you want.
First of all let's look at exactly what acid is. In chemistry it is defined as a substance capable of forming hydrogen ions when dissolved in water. Acids can weaken cellulose in paper, board, and cloth, leading to embrittlement. Acids may be introduced in the manufacture of materials and left in intentionally (as in the addition of certain sizings, like starch or glue) or incidentally. Acids may also be introduced by migration from other materials or from atmospheric pollution. Discoloration and embrittlement are attributed to acid.
Believe it or not, the natural oil on your hands are actually highly acidic, and merely handling your photos can transfer this "acid" to them The best and safest course of action is to keep your photos in acid free photo albums and once you create a layout for your scrapbook to encase it in an acid free page protector.
Materials that have a pH of 7.0 or higher are acid-free. Many manufacturers are using the term PH Neutral instead of acid free. PH Neutral is more definitive than acid free. An acid free product could be extremely alkaline. Since is is also undesirable to be highly alkaline because of damages, PH Nutral is a desirable quality.
Archival-quality is a term used to indicate that a material is chemically stable, and therefore, has a stronger resistance to adverse environmental conditions. Archival quality suggests that a material or product is permanent, durable or chemically stable, and that it can therefore safely be used for preservation purposes. Archival standards are the principles that most museums adhere to.
Lignin is a component of the cell walls of plants that occurs naturally, along with cellulose. Lignin is largely responsible for the strength and rigidity of plants, but its presence in paper and board is believed to contribute to chemical degradation. It can be, to a large extent, removed during manufacture. Currently most photo preservationists believe lignin to be more harmful to photos than acid, although additional research is needed to determine the precise role lignin plays in the durability and permanence of paper.