Over time the level of Polyethylene Glycol in the solution that the timbers sit in has been gradually increased.
As the Timbers sit in The PEG solution, the water trapped inside the wood is gradually extracted and replaced by the waxy Polyethylene Glycol.
In order to continue extracting and replacing the water in the wood, the ratio of PEG to water must be incrementally raised.
In total this process can take up to 3 1/2 years for some of the larger timbers