TRANSPARENCY / OPACITY
Paint that is transparent will allow light to pass through it. You will be able to see through transparent paint. Opaque paint will absorb and reflect light. This paint you will not be able to see through. Most manufacturers indicate transparent, semitransparent or opaque on their labels.
The drying time is not printed on paint labels. Different pigments have different drying times. Some oil paints can be touch-dry in hours, others can take weeks (depending on thickness of paint, dilution, and ground absorbancy).
Pigments come from many sources, such as minerals and metal oxides, or from organic
Sources, such as plant and animal substances. There are many synthetic versions that equal, or even improve on, most of the characteristics of the natural pigments.
Medium is the term for the fluid that surrounds the pigment and facilitates its application onto a surface. In the case of oil paints, the medium consists principally of the binder and the thinner.
If paint is diluted with oil alone it may wrinkle, crack, become yellow, or take too long to dry. So a thinner is mixed with the paint to help aid the flow and drying time.
Varnish is a protective coating for the painting. A varnish can be matte, gloss, or something in-between. Varnish paintings six months to a year after completion, depending on the thickness of the layers and types of pigments and mediums used. After many years of exposure to the atmosphere, the varnish may have absorbed substances that darken or discolor the appearance of the picture. In this case it can be removed and replaced.