Exactly what I came in to mention. The quality of sound produced by the ‘golden age’ Strad’s has never been reproduced and no one knows how to do so.
Joseph Nagyvary spent years doing research, but has not matched the quality of Stradivarius. Though he claims that it would be possible.
Another thing to consider: AIUI there are census data cards from the late nineteenth and early twentieth centuries that either are unreadable, because the machines for reading them couldn’t be maintained properly or because the algorithms for reading the data have been lost.
For that matter, for another modern example of something that we can’t economically reproduce: NYS voting machines - the mechanical punch card ones. They work quite well, to this day. The machines that the workers can keep in service, that is. To keep the whole inventory of machines in service is predicated on an existing tool-and-die industry that just isn’t there anymore. AIUI, one reason that NYS is in such a mess for replacing voting machines is because no one really wants to do away with these dinosaurs* but the cost of keeping the skilled workers and tools shops in business are prohibitively high.
*I’m maligning the voting booths, by calling them dinosaurs. They really do work well, and make for a recountable, auditable recorded vote, with a minimum of opportunity for voter confusion. It’s just that their time, through no fault of their own, has passed.
I play old time (1920s and 1930s) music on the guitar. Some guitar techniques used on scratchy old recordings still baffle pickers today. Others have been figured out, after lots of attention from lots of people.
It’s been an education for me how easily craft details can be forgotten if they aren’t captured in still or moving pictures.
Dude, what History Channel are you watching? Every time I turn the danged thing on, it’s a program about Hitler’s doctor injecting him in the buttocks with meth. Which, except for the current unavailability of Hitler’s butt, is an eminently re-creatable phenomenon. 
I think the point is that the Japanese fellows went to England or wherever to learn the technique, rather than to show our boys how it’s done, and all that.
The resident I talked to was very proud of the house’s history and was very gracious about showing me the interior. This was 20 years ago, though, and I don’t know who lives there now.
Today, we can copy or even make better Damascus Steel than they could “back in the day”. There’s quite a few mastersmiths who would be happy to take $1000 off your hands for a wonderful piece of steel art.
Greek Fire, now- not so much.
wiki"The ingredients, process of manufacture, and usage were a very carefully guarded military secret—so secretive that it remains a source of speculation to this day. Speculations include
* petroleum, niter, sulfur;[9]
* naphtha, quicklime, sulfur;[10]
* phosphorus and saltpeter.[11]
It is not clear if the operator ignited the mixture with a flame as it emerged from the syringe, or if it ignited spontaneously on contact with water or air. If the latter is the case, it is possible that the active ingredient was calcium phosphide, made by heating lime, bones, and charcoal. On contact with water, calcium phosphide releases phosphine, which ignites spontaneously. The reaction of quicklime with water also creates enough heat to ignite hydrocarbons, especially if an oxidizer such as saltpeter is present."
Zsofia: I have a friend who is a quite accomplished flintknapper, and he sez there are dudes alive today who can do anything the ancient could. Myself, I can make something sort of sharp… that would make a Neanderthal 8yo laugh his ass off.
There is a wisp of memory floating around my fuzzy little brain about a paint color (a certain shade of red?) in Renaissance art that is unduplicatable today. Seems like not only was the exact recipe a closely guarded secret, but the main ingredient (some specific variety of sea critter?) is now extinct.
My google-fu is weak, but if my memory is even semi-accurate, that may be something that truly can not be duplicated today.
That sounds somewhat similar to the chilazon, a rare sea creature that produced a blue dye used in the making of a blue thread for the tassels of a Jewish prayer shawl and other garments. The knowledge of how to make the dye, and the identity of the chilazon, was lost in the ancient past.
I posted a similar question years ago - Have we lost any technologies? There were some interesting responses. I think the conclusion was as posted in this thread - in most cases, we have no need or care to keep the knowledge current.
Four years ago, I took a month’s vacation in England. Part of my trip was a visit to Hadrian’s Wall, where I was lucky enough to fall in with a local archeology professor leading his class on review before exams. At one point he was talking about the earthen walls that formed part of one of the forts, and he mentioned that they’re still trying to figure out how the Romans built those walls which are still pretty danged vertical after 1,000 years. So far, all modern attempts at duplication wind up slumping within a short time.
I wonder if they’ve figured it out yet.
Actually, many anthropology students learn it - and after you get the nack, it isnt that hard, There is a whole subculture of recreationsts who learn to do similar ‘ancient tech’ to do things. I can light a fire using flint and steel, fire bow and rub 2 sticks together styles, make a flint tool, cook over a fire, take a dead animal and render it into food [though I am not good at tanning, or traping. I can hunt and fish] and this is just from interest. My area of research in teh SCA was food history, western steppes culture and a side swing into early imperial roman womens life.
One that we talk about all the time in theatre history is the Mechane. You know, the crane that lowered an actor portraying a god in Greek theatres from whence we got the term “deus ex machina.”
We can’t make one like it today. We have no idea how the Greeks managed it and there is even some suspicions that it might not have really ever existed.
Anyway, I agree that the phrase usually means “we have no clue.”
A purle dye extracted from a Mediterranean shellfish (the murex shell) was used by the Romans to mke their imperial purple cloth. This color could only be worn by the upper classes. Many eyars ago, a couple of HS girls did a project on this, and succeeded in duplicating the process.
LOL it isnt hard, per se … just really REALLY disgusting…
more spiffy info on teh jewish version
And down towards the bottom how to make it
My only comment will be that I can only imagine the stench of a murex fermentary sited next to a linen rhettery … :eek: 
Aruvqan, where are the the primitive goodies archives? The link is dead; not the one you gave, but the link from that page to the archives.
Hmmm, last I heard the so-called uniqueness of Strads et al had been shown to be the listener’s perception, as they knew they were supposed to hear something special.
In fact a “blind” test mentioned here has a modern copy of a Strad winning out over a genuine one.
Next to the woad dyer - you know - down tannery street - opposite the abbatoir and glue works.
What? Wouldn’t it make more sense for them to be near the fish drying racks?