After remaining static for many years there have been a number of changes in plant design and treating cycles in recent years. This has been particularly true in the USA where few restrictions are placed on plant treating cycles by specifications; since only results type specifications are used. It is also important to realize that the AWPA Specifications for Southern Yellow Pine only call for treatment of the sapwood since the heartwood has a high natural resistance to termites and decay. This is evidenced by old plantation houses on Southern and West Indian sugar Plantation houses that have stood for several hundred years. This paper attempts to set out these changes and the reasons for them. Industry often appears to have jumped ahead of research or the results of research have not filtered down to the industry and these knowledge gaps are mentioned in the appropriate sections. These sections try to separate the many inter-related factors into simple headings covering plant components and other factors influencing treatment. Some of the criteria presented in this paper have only been recently recognized as of importance so that results from past research is often found to be inconclusive when studied under the light of present day knowledge (e.g. rate of pressure rise was not noted).

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