Film is made up of several microscopically-thin layers. For the purposes of this project, it is helpful to know about the two most important: the base layer and the emulsion layer.
The emulsion layer is the layer that actually contains the image. This side has a matte-like appearance.
The base layer is the transparent, supporting layer to which the emulsion layer is adhered with a binder. In our microfilm collection, the base layers are likely to be of an acetate variety or polyester. It is unlikely but possible that you might encounter a very old microfilm with a nitrate base. The base side is noticeably glossier than the emulsion layer.
One common problem with deteriorating acetate-based film is that, as it chemically breaks down, the base shrinks, but the emulsion layer does not. The result is that the film base pulls away from the emulsion layer, which can look like bubbles between the layers or flaking off of a brittle, yellowed layer (which is the base layer). In extreme conditions, the base layer becomes so brittle that attempts to flatten it can cause the base layer to break.