Tacks and wood, this needs explaining, please

I was wanting an old style wooden screen door for the back of my house. Found and bought a beautiful old door, all the curlicues and it’s green. It was covered in over a quarter inch of dust, it was hard to see parts in the photo. Just what I was looking for.

I begin to disassemble it, removing the brittle covering strips to remove what remains of very old disintegrated screening. I don’t know how old it is, but I could see originally the screen had been splined in. Then, at least twice screening had been tacked in with something resembling carpet tacks. I decided I would remove the tacks that were standing proud, as it were, and leave those that were flush, in place.

The wood is remarkably hard, some of the tacks were truly difficult to get free. On the other hand, about half of them, as soon as they were touched with the tool, came out like they’d been shot out of a gun. Whizzing across the room. And when I say, ‘…like a shot out of a gun!’ I mean it literally made a loud cracking bang, like a gun. Loud enough hubs came out to see what was going on.

What the hell was that?

Well, I can explain the hard word part, at least. Wood hardens as it ages due to the resins in the wood turning into something like amber. Except stronger, because it’s a composite with all the wood grains still running through it.

Which might also explain the whizzing. It’s possible that the metamorphosis of the wood increased the pressure on the tacks, to just slightly less than the maximum force the friction on the tacks could withstand. And one thing about friction is, it’s stronger on things that are stationary with respect to each other than things that are moving. So when you just barely started moving them with your tool, suddenly they were only held back by kinetic friction, not static friction, and the pressure was easily able to overcome that.

Some tacks are tapered so if they are under a little pressure as soon as you get them moving they suddenly go off.

I agree with the posters above that, for the reasons they explain, at least some of the tacks were “spring loaded” to shoot out.

The proud ones had probably started out flush but over time had been slowly extruded partly out by the same “spring” pressure. I also bet that a careful look at the wood would find a hole or two where a tack used to be. Which had completely self-extracted over time.

In addition to @Chronos’ fine comment about wood hardening over time there might be another factor given the age of the OP’s door. Which would not go to the “spring-loading” feature but to the OPs general observations about the wood’s quality.

Prior to about WW-II, a lot of the wood used for ordinary construction and moldings and such was older growth tightly grained “high quality” wood of various hard species that grew naturally in the USA’s forests. Since (very roughly) WW-II, the vast majority of construction and molding wood is soft wood planted in tree farms where the species is selected for growth rate and yield per acre per year, not for strength, density, or hardness. It’s of tolerable quality for its mission: cheap, easy to shape, and hidden inside walls or behind paint.

A DIYer used to working only on wood bought in the last 30 years from the likes of Home Depot might be quite surprised at how different even ordinary 2x4s were back in Ye Olden Dayes before slapping up suburbia ASAP became a big business and before the USA had cut down most of its old growth.

They just weren’t income tacks…nyuk…nyuk…nyuk.

When I remodeled a century home and needed to cut out some framing to run furnace ducts I had to use a chainsaw. That 100 year old southern yellow pine would just smoke if trying to use a Sawzall.

For one thing, 2x4s were once actually 2x4.