I am pretty sure they used creosote not arsenic to protect the timbers, same as all light poles have on their buried section. then I may be wrong.
It goes back to the Greeks and can be found (dark stains) on almost any old wood structures particularly bridges as weathering protection, rot protection and insect protection.
Coal-tar creosote is the most widely used wood treatment today; both industrially, processed into wood using pressure methods such as "full-cell process" or "empty-cell process", and more commonly applied to wood through brushing. In addition to toxicity to fungi, insects, and marine borers, it serves as a natural water repellent. It is commonly used to preserve and waterproof cross ties, pilings, telephone poles, power line poles, marine pilings, and fence posts. Although suitable for use in preserving the structural timbers of buildings, it is not generally used that way because it is difficult to apply. There are also concerns about the environmental impact of the secretion of creosote preservative into the aquatic ecosystem.
When pine logs, fencing and poles started replacing hardwood in Australia in the 60/70 period they were treated with a green copper arsenate but this proved too dangerous, particularly if people used the wood to burn so the green treatment of pine logs today is an updated formula of various chemical salts.
Most pressure-treated lumber sold before January 2004 was treated with chromated copper arsenate (CCA for short), which contains arsenic. ... Manufacture of CCA-treated wood for residential use was halted December 31, 2003
I think because it burns if you get it on you and is slightly carcinogenic. Easy to apply to the timber before assembly but difficult in small completed structures such as buildings or the chippies would be handling coated wood. Can't be painted successfully afterwards which most buildings require. Not nearly as bad with bridges and light poles where pretreatment is easy and even afterwards on large outside areas.
The big beams and piers are generally solid under the grey weathered external.......the old timbers were also saturated in arsenic to stop rot and termites ......the weathered timbers that are splintered are epoxy impregnated to make patterns for upmarket kitchen and benchtops............guy at Willawong was showing me an order he was getting together for an archtect ,cost was over $100,000 ........The timbers are also full of huge steel bolts and straps ,which have to be carefully removed ,no oxy cut,that would spoil the iron stains in the wood.
Clients seem to love leaving all those bent bolts and straps on the timbers. Gives it charming character they say.
I remember restoring the flagged verandahs of the old St Aubins Inn at Scone a few years back. All the original posts were rotted at the bottoms. Owner wanted new but we came up with making plywood casting boxes over the damaged ends and added an epoxy filler mixed with matched hardwood shavings to repair each post.
Similar thing can be done when a mistake is made when carving stone only using colour matched fine sandstone dust to the epoxy.
In order that the labour of centuries past may not be in vain during the centuries to come... D. Did
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