For "ex situ" processing, oil shale is crushed into smaller pieces, increasing surface area for better extraction. The temperature at which decomposition of oil shale occurs depends on the time-scale of the process. In "ex situ" retorting processes, it begins at and proceeds more rapidly and completely at higher temperatures. The amount of oil produced is the highest when the temperature ranges between . The ratio of oil shale gas to shale oil generally increases along with retorting temperatures. For a modern "in situ" process, which might take several months of heating, decomposition may be conducted at temperatures as low as . Temperatures below are preferable, as this prevents the decomposition of limestone and dolomite in the rock and thereby limits carbon dioxide emissions and energy consumption.