Firstly, I'll just say I love my customers, partly because they do things like this....
Southdown is meant to be a great wool to use for socks, all the down breeds resist felting, so in theory that also means you can machine wash your socks. Handy, if like me you do minimal laundry and tend to want to put everything in together.
You can buy machine washable wool, it's superwash treated, either by being chemically stripped of the scales that stick together during felting, or by being resin coated. Both require the use of chemicals, and the resin treatment in particular doesn't seem to last forever. I have, eventually, felted socks that were meant to be superwash wool. That's not to say I'm anti superwash, I'm not particularly anti-chemicals in modern life, I particularly love the intense shades I can dye on superwash, the stuff sucks up dye like nothing else. However, a natural alternative that's also machine washable is a pretty cool tool to have at our disposal.
Now like I said, Southdown is machine washable in theory, but I'd never personally put it to the test. Projects to do outweigh available time unfortunately.
Now I won't need to, as sockrat has done it for me.
She knitted 2 swatches in some of my hand dyed Southdown. The colourway was autumn hedges from last year's BoB club. Look closely and you'll notice that one has a little strand of red yarn knitted in to it, this was so she could tell the difference between the two swatches.
Both swatches originally measured 5 inches wide by 3.75 inches tall.
The one with the red yarn was treated delicately, just a cold machine wash. The other one got some abuse, same cold machine wash, but then put straight in a hot dryer.
They had 4 washes/drys, and both swatches did change shape slightly.
Here's the delicate treated one (cold machine wash only)
It's filled out slightly, developed a bit of a halo, but barely altered. It ended up measuring 5 inches wide, and 3.6 inches tall.
Here's the one that was put in a hot dryer while still wet.
It's definitely fuzzier, but it's not exactly a square of felt, it definitely still has identifiable stitches. Finished measurements were 4.5 inches high, and 3.5 inches wide, so slightly more shrinkage than the other swatch, but not by a great deal.
I have to say, we don't really tumble dry anything, it either goes on the washing line outside, or is hung on the airer above the woodburner overnight. Sometimes it takes a combination of the 2, but our tumble dryer hasn't been switched on in a very long time. Having seen this I feel perfectly safe putting my Southdown socks in with all the rest of my washing on a low temperature setting. It seems to work just as well as superwash treated wool. If you do choose to use a tumble dryer I'd be inclined to avoid drying your Southdown socks in it, but if you do accidentally put a pair in with the rest of the washing it's not going to be the end of the world.
Of course, from a dyers perspective hand washing does make the colours last longer, they stay brighter for longer, but if you know you won't handwash your socks then Southdown is a really useful fibre to have in your spinning tool box.
As for how to spin it, Sarah Anderson did another experiment in her Spinners Book of Yarn Design. She spun up several different types of yarns and made them in to pairs of socks, and carried out wear tests. The book itself is well worth buying, Christmas is coming, why not put it on your list...
Traditionally the gold standard for socks is a 3-ply yarn. She compared chain or navajo plying to a regular 3-ply and debunked the myth that navajo plying makes a weaker yarn. So go ahead and navajo ply if you want.
She also experimented with opposing plys, which were better still than 3-ply, and cabled yarns which were also better, for that technique you need to be able to produce fine enough singles to make the right thickness of yarn with 4-plys though...