Collodion has some unique properties that make it well suited as a carrier for silver salts. It's soluble in ether and alcohol--two solvents with low boiling points and high vapor pressures. This allows the collodion to set and dry quickly. Once the collodion is set, it forms a stable permeable layer that can be sensitized, exposed, developed, and fixed. When the collodion dries, it forms a very thin, tough layer--impermeable to aqueous solutions. Collodion is also extremely sticky--it will adhere to just about any surface. Gelatin and gum arabic are soluble in aqueous solutions, so a surface coating will dry much more slowly.