Silver has been used for thousands of years as ornaments and utensils, for trade, and as the basis for many monetary systems. Of all the metals, pure silver has the whitest color, the highest optical reflectivity, and the highest thermal and electrical conductivity. Also, silver halides are photosensitive. Owing to the above properties, silver has many industrial applications such as in mirrors, electrical and electronic products, and photography, which is the largest single end use of silver. Silver's catalytic properties make it ideal for use as a catalyst in oxidation reactions; for example, the production of formaldehyde from methanol and air by means of silver screens or crystallites containing a minimum 99.95 weight-percent silver.