OT tack alloy in 1912?

[mccloud]

[Though there are no halos around any of the tacks, the screws holding the keel on were about half-length, corroded away, with almost nothing left of the cone washers.]

The above is a quote from the post I made on 11 Aug regarding OT 20147. Weeks later the little lightbulb flashed on with this thought: "No zinc, no dezincification". The original tacks used to build OT 20147 in 1912 have a very dark red appearance, closer to silicon bronze than to the bright golden brass canoe tacks we use now, and those old tacks are just as solid as they were 100 years ago. So what was the composition of the tacks that Old Town was using in 1912? Just copper, copper/arsenic brass, some other brass alloy lacking zinc? And those screws which had been used to hold the keel on must have been a copper/zinc alloy, and so were badly decomposed. Any other thoughts? Tom McCloud

 

[Andre Cloutier]

red brass maybe?

 

[Benson Gray]

I am not a metallurgist and Old Town appears to have used tacks made out of a variety of different metals over the years. Your assumption that this 1912 canoe has no zinc in the tacks seems reasonable. Good luck with the restoration,

Benson

 

[mccloud]

They could be red brass of some composition. One of the alloys called red brass is around 88% copper, 8–10% tin, and 2–4% zinc, while another is 84–86% copper, 0.05% each iron and lead, and 12-14% zinc. Both of these are supposed to be more resistant to decomposition in salt water than copper/zinc brass. Other alloys of brass contain no zinc at all. Of course the exact composition of the alloy would have been of little interest to Old Town, so long as the tacks were strong enough to be driven into the wood and hold it together. As Benson suggests, the tacks and screws came from different sources over the years and their composition varied. Determination of the composition of the brasses used in canoes over the years would make an interesting research project, for example, for a summer student in the chemistry department at the University of Maine where there is access to an atomic absorption spectrometer. Tom McCloud

 

[Benson Gray]

The factory inventory from January, 1911 shows 6281 pounds of 11/16 inch "copper tacks" and 144 pounds of 3/8 "copper tacks" that were both about $0.19/pound along with 35 pounds of two and a half ounce (i.e. 1/4 inch) "brass tacks" that were around $0.25/pound. There does not appear to have been a detailed physical inventory in 1912. The one from January, 1913 shows a similar assortment of tacks and the prices had increased slightly. (See the modern prices for tacks at https://www.wooden-canoes.com/canoes/materials/hardware/ if you want a good example of inflation over the past century.) Both of these pages are attached below if you want more details. This still doesn't provide any detailed alloy information but it may further confirm your original idea.

Benson