Sewing machines were necessities because people made, kept and mended all their own clothes.
But the OP says “What one item from your house would medieval people recognise as something immediately worth a fortune?” I don’t think that people would immediately recognize the value of history since they wouldn’t know it to be accurate. And then there’s the grandfather paradox so it might not even be accurate.
According to wikipedia, nuclear submarines never need to be refueled throughout their 25-year lifespans. The reactor could be used as a source of power for all the electronics and other devices on board (including of course a computer with an extensive database of modern knowledge). The submarine could be also used to grant an absolute naval domination worldwide, as well as bringing tomatoes, potatoes, and, uh… spices, to the old world. As for the crew, I’m sure a local lord would be more than willing to provide a group of hard-working and talented young serfs to man the vessel. Also, there would be an armory full of modern firearms on board.
I really have to think that a nuclear submarine is a household item for vanishingly few of us.
Basic tools were seen as items for mechanics, which is to say, the lower class. They would be salable, but not for the prices that the upper classes could pay.
Binoculars are a good idea. Their military value would be immediately evident, and they would command a high price. I would put them up there with wristwatches and atlases.
What about sugar or salt? I have a 5-lb bag of sugar in my cupboard.
I would bet that some high yield seed grains that are resistant to disease would be a winner - maybe some magic beans.
They didn’t have potatoes, so that might be a start, couple of tons of those might be a good start.
A couple of counterpoints to your revolver suggestion:
Hardiness: Possibly because revolvers are older than semi-auto pistols, they’re assumed to be more reliable. In reality, the machinery necessary to move the hammer and cylinder, and to time it all properly, is comparible to clockwork. A rugged semi-auto like a CZ pistol or a plain ol’ M1911 will probably outlast your average revolver, and not require a master gunsmith to fix timing issues.
Accuracy: Accuracy beyond say 25 yards isn’t a major concern when talking about handguns. Accuracy with a handgun is going to be 95% operator skill anyways.
Stopping power: This is more of a function of the round, and not the pistol style. Besides, revolvers like the .44 Magnum and .500 Nitro Express are just about the baddest bruisers out there.
One modern gun is an attractive trinket, but wealthy people already had attractive trinkets. You need to offer them something that is going to increase their power and influence if you really want to parlay an item into wealth. Whether we aren talking guns or sewing machines or timepieces, you need something they can copy at their existing level of tech, they will understand well enough not to fear, and that will improve their leader’s position somehow. Heck, a garden weasel is a good candidate in a subsistence agriculture economy. It might take the skill of a swordsmith rather than a village blacksmith to produce them, but an increase in productivity would translate to increased wealth and power for the despot in question.
Otherwise, just take all your cheap ass junk jewelry back in a pillowcase and sell it to them as genuine.
Hm, picking for the hell of it the year 1400, and the location Southern France. I will assume that mrAru and I have been run through a medical machine returning us to full health at the specified age of 30. We would be dressed in the correct traveling clothing for pilgrims, I would take back a backpack with enough worn gold and silver coins, pilgrimmage tokens and a french-english dictionary [I have not been using my French and would need it until I am up to speed], a breviary [not being Catholic, I would need a ‘cheat sheet’ to seem like I am saying the proper prayers at the proper time] and an ‘almanach’ suitable for the decade after 1400 with information on the movable feast dates and planting information. Adding in a book of receipts that include recipes for interesting and useful chemicals and medications, and having had a full set of innoculations. mrAru would carry a backpack loaded with a selection of new world based seeds, old world based seeds, potatoes of several kinds [mealy and waxy] and a couple of other more mechanical based reference books with drawings on how to make certain things. Finishing up with a couple of nice mules broken to ride and plow we would be good to go to find a small plot of decent land to farm that we could rent. With some of the gold we would hire a couple men to help mrAru with the farming, and their wives to help me with the girly jobs [while I can spin, I really hate it.]
The new world specific crops would simply be farmed and seedbanked, though potatoes could reproduce fast enough to be of limited use in the same year. We could claim that the odder plants we brought back from pilgrimmage with us. Being fellow farmers rather than aristos we could probably get potatoes, tomatoes and peppers into the diet a bit easier than the aristo explorers did. Being willing to live simply and rent a farm instead of a big flashy get rich and climb the social ladder that is the mistake of may time travellers may make it easier to ft in. I would also rather go for somewhere warmer than Britain in the little ice age.
In one of the 1632 books, a guy who was pulled back in time with his town had the great idea of buying up all the RIT dye in town before anyone else thought of it. He bought locally-made cotton thread and dyed it in all the amazing modern colors, then sold it to embroiderers at very high prices. He was the 17th-century equivalent of a multi-millionaire in a couple of months.
Of course, a bunch of dye packs don’t fit the challenge: You’d need to prepare ahead by gathering a lot of it before your trip, and it would take a while before you could turn it into the immediately-recognizable valuable product.
How about a 50’ garden hose or other type of drainage hose? It could be used to siphon water (or sewage), clearing flooded cellars or fields. Combined with a simple hand pump, you could do a lot of custom work - you could be the medieval Roto-Rooter.
A large book of knitting and/or crocheting patterns.
I don’t believe anyone has mentioned a compass yet. Those that existed were hardly portable and available only to the most select few. With a few explanatory words to introduce its use, you could open up a whole new world to your 14th century hosts.
I wonder what the cost was for black pepper back in those days? I know I read before black pepper was a valuable spice but what value, I cannot find numbers.
Incredibly dangerous, as you note. People were being burnt at the stake for translating the Bible into English in the 16th century, long after the medieval period. A compendium of the classics would be a very good idea, given the value they placed on Greek ideas an encyclopaedia on Galen, Herodotus, Aristotle etc. would be highly prized.
Not that I disagree, but the fervour over killing witches is somewhat overstated. The big trials were centuries later, but it was pretty hard to get yourself killed as a witch or wizard - it was only illegal to cause harm by black magic or summon demons.
Wrong horseman of the apocalypse. Plague, or the Great Mortality as the contemporaries would call it, hits Europe in the late 1340s. Famine hits hard circa 1315-17, though.
Thanks for the replies so far. Had another thought - soap. The finest cakes of white soap would be left in the dust by even the cheapest shower gel you could get your hands on, and again it has the value of being easily demonstrable and understood.
ETA: Pepper is a damn fine idea too. Haven’t found specifics on pricing, but apparently the cost was so high it was one of the factors prompting Portugal to explore India and gave the Dutch the expression “pepper expensive”.
I’ve always heard, although it admittedly could be a UL, that bathing was not a common activity and so the value of soap might not be as valuable as one would think.
I disagree. I make home made soap out of lye and common oils (olive oil, lard, etc) and it is vastly superior to store bought stuff. Granted, I have accurate measuring tools so the product is more consistent than you’d likely find in the middle ages, but it would be similar.
Is this a one time sell it and go home scheme, or do I have to stick around and face the consequences once my battery-powered devices loose their power?
Hmm. Maybe not then, although I suspect the quality of your ingredients and accumulated knowledge would make your homemade soap (first, we render fat…) better than what was available to even nobles.
You just have to get rich - and quick.
Well, if we don’t have to worry about power beyond the initial battery life, I could see an argument for a laptop or tablet. I think it would look magical enough that people in power will want to control it, and by the time the battery dies, you’re out of there.
According to Wikipedia streptomycin, gentamicin, tetracycline, and ciprofloxacin are all effective against bubonic plague and these days none of those are particularly exotic. The real trick is getting them into a person in time to do some good.