Oil shale gas is produced by retorting (pyrolysis) of oil shale. Oil shale retorting followed in 1881 by utilization of "D" retorts. Several co-pyrolysis processes to increase efficiency of oil shale retorting have been proposed or tested. To increase efficiency when retorting oil shale, researchers have proposed and tested several co-pyrolysis processes. The ratio of oil shale gas to shale oil generally increases along with retorting temperatures. Alberta Taciuk Process technology extracts bitumen from oil sands through a dry retorting. The plan was to produce shale oil by underground mining and surface retorting. Several companies have patented methods for "in-situ" retorting. Yields of retorting are considerably higher than those of kilning, and may reach 35%-40%. Halonen took the incident in good humour, retorting that Berlusconi had "overestimated his persuasion skills". Jonah is starkly against this plan, retorting by saying he is fine. In canned seafood, it is used to maintain color and reduce purge during retorting.