The split-toning process is used elsewhere on this site, but features to some extent in these infra-red pictures also.
Some were made using Kodak Infra Red 35mm film and all were printed on the magnificent Agfa Record Rapid paper, now sadly no longer manufactured. In most cases the toning is allowed to proceed to completion, giving a rich warm brown.
Nikon F, KodakTri-X, printed on Agfa Record Rapid, toned in Kodak Selenium Toner