Thanks to Linode Cloud Computing, for  supporting this episode of SciShow.  Head to linode.com/scishow  to learn more and get   a $100 60-day credit on a new Linode account. [♪ INTRO] Knowledge isn’t forever.

History is full of things like personal histories, languages, and even whole technologies and   techniques that we no longer remember. But one of the coolest things about archaeology  is that it gives us a chance  to re-learn things.

So here are four examples of really  cool technology we kind of lost, how scientists are using modern techniques  to study and re-learn them, and, ultimately, why these techniques  nearly disappeared. One of the first examples of impressive  ancient engineering is Roman concrete. Rome built some truly epic buildings, and  many of them are still around today, like the Pantheon.

Roman concrete was incredibly durable,   especially when we’re talking about its use in marine environments,   like building harbors. Building something that can last thousands   of years in wave-wracked seawater is truly impressive.   But with the decline of the Roman empire,  people seem to have more or less   stopped using this super durable  concrete on a wide scale.   And for a long time, we didn’t really know  what made this concrete so durable.   I mean, we knew just a tiny bit. For instance, thanks to recipes   recorded by people like Vitruvius, historians knew that the Romans used   a different recipe than modern concrete.

Concrete, in general, is a mix of aggregates,   like little bits of rocks or brick, water, and cement, and varying that recipe   can change its physical properties. The Romans used a particularly   involved mix of aggregate along with lime, seawater, and volcanic ash.   But not just any ash: It was a  special kind called pozzolana,   which was dug up near Naples. This mix seems to be special,   since people would actually pay to import this ash when they were   building stuff.

But we didn’t know   what actually made this mix special. In a study published in 2017, scientists   decided to find out. They got samples in the   form of long, cylindrical cores that had been drilled out of Roman ruins   and then used a microscope and X-rays  to look at the minerals inside.   They found that the answer to the  incredible durability of Roman concrete  lies in the chemistry inherent in the concrete, in large part thanks to the mix of both seawater   and that special volcanic ash.

Silica and aluminum oxide from the   pozzolana reacts with lime and the seawater to create crystals of a mineral called   tobermorite, which reinforces the   concrete and makes it less likely to crack. Even better, further exposure to seawater causes   the growth of more tobermorite, explaining why Roman concrete did   so well in coastal projects. But if this was such cool tech,   how did the whole world just kind of seemingly forget about it?   Well, in this case, the answer  partly has to do with demand.   In short, with the collapse  of the Roman empire,   the nearby demand for this kind  of mega-engineering went away.   It no longer made sense to  build quite as grandly   or to pay for the special pozzolana  to be dug up and shipped.   With less demand, the knowledge  of how to make this concrete   started to fade away.

It didn’t entirely disappear,   we have examples of engineers still reaching for pozzolana long after the Romans fell,   but it may have become more of a trade secret or kind of oral memory   rather than a codified recipe. It was only later, in the 1700s,   that science kind of formally recognized and “re-discovered” it again. The moral is that technologies  exist in context.  When it made sense for Roman concrete  to be widely used, it was.  When it wasn’t, it faded into memory.

Speaking of being widely used,  Roman concrete might make a further  resurgence sometime soon.   A number of scientists have suggested that  the formula, or something similar to it   using coal ash, might be a good  candidate for marine uses today,   and produce fewer carbon emissions. Now onto something softer…Dhaka muslin   was a thin, high-quality fabric that came from what’s now Bangladesh.   It had been produced for centuries before  eventually becoming a big hit in Europe   in the late 1700s. The material was made   of a special variety of cotton that grew along the Meghna River,   which had very short, easily frayed threads.

This made the material kind of a beast to   work with and turning it into fabric required a convoluted, 16-step process  that would have required multiple households. One step could only be done on boats,   for instance, as the material needed to   be a certain level of humidity. The payoff of all this work,   however, was incredible.

The material was very light and thin.  It was said that you could fit 18 meters of  fabric into a pocket-sized snuff box and it   could apparently reach thread counts  around 800-1200 threads per square inch.  But by the 1900s, it seems to have disappeared. So why’d we stop making it?  In short, because the British  East India Company or EIC   made it impossible for farmers  to make a profit off of it.  In the 1700s and 1800’s, the EIC effectively  took over large portions of India  and started to muck with the system. One thing they did, for instance,   was to put the farmers into terrible financial situations where it was really   easy to go into irrecoverable debt.

In the meantime, British manufacturers flooded   the market with inferior look-alikes. The end result was that the farmers,   no longer able to make a stable living  off it, abandoned the practice.   And without people to continue growing it, the variety of cotton it was based on went   extinct. By the 1900s   Dhaka muslin was a thing of the past.

That said, scientists are trying   to renew the industry. Luckily, the technique was   written down, so we do know how to make it, but we still need that special cotton.   And although it doesn’t grow anymore, we  may be able to kind of recreate it.   And it’s all thanks to leaves  preserved at the Kew Gardens.   Scientists could extract  the DNA from those leaves   and form a genetic signature for this plant. Then it was off to the races, they started a   scavenger hunt for the closest wild plant near that one river and found one that’s a   potential cousin or descendant.

Today, they've grown those plants   and are making something close to the original garments again.   So what can we learn from Dhaka  muslin’s disappearance?   Again, it kind of comes down to economics, but in this case, specifically, trade and   colonialism were disrupting the industry. We can see this in other places too.   The Haya in Africa had a unique way  of making a very high-quality steel  and were doing it long before many  other people had the technology,   but the technique was nearly lost in the 1900s   when cheap European steel flooded the market. So, in both of the last two points, the answer for why something was lost  kind of boiled down to economics.

Sometimes, however, techniques are lost because they were too secret to spread around…   If you look at the history of  warfare, one of the most mysterious   and terrifying weapons ever invented in  the past was probably Greek Fire.   It was introduced by the Byzantine  Greeks in the 600’s CE.  And it was described as a liquid that was  discharged through a tube or grenade,  which could burn on water with  thunderous noise and a lot of smoke. There have been many proposed explanations  for what this liquid could be,   but we don’t know exactly what it was  because its identity was a secret.   It was a military asset, so the  people who knew how to make it   probably didn’t want their  enemies learning the process.   Figuring out what it may have been generally  boils down to meticulous research   of existing historical records for clues and comparing proposed concoctions to see if   they match the expected properties. For instance, people have proposed   it’s based on saltpeter, kind of like an early gunpowder,   but others think it was probably based on crude or refined petroleum, something   like naphtha or napalm.

In this case, the people who   made this stuff purposefully didn’t want a lot of people to know how it worked.   It’s been suggested that it may have  been whipped up in a central lab,   sealed in containers, then  delivered to the front lines.   So soldiers using it wouldn’t have  ever known exactly how it was made.   Even if it was captured by the enemy,  they wouldn’t know how to make it.   It's also been suggested that it may have  been pretty complicated to actually use   and needed a whole system to make it work,  like how to prep it, heat it, and use it.   If true, it's plausible that the process  could have been compartmentalized,   so even if you knew part of the system, you still wouldn’t be able to   figure it all out by yourself. But the secrecy meant that the knowledge   of how to make this was tenuous. Some scholars suggest that only a   small number of individuals understood the whole system at any one given time.   And, eventually, as regimes were overturned  and people died, the secret was lost.   So the reason the tech was forgotten  was mainly due to secrecy.   And we can see this with a lot of  proprietary or secret stuff, even today.   In the early 2000s, there was  apparently a bit of a snafu when   the US military needed to  refurbish their nukes,   but there were problems figuring out  how to get more of a substance   code named “fogbank” because only a  handful of records had survived   and most of the folks who knew  how to make it had retired.  Now for something less terrifying.

Painted on some of the terracotta soldiers   in China was an unusual, bright lavender pigment,   which we now know as Han purple. An interesting feature because at   the time it was being made, from around 800 BCE to 220 CE,   purple and blue pigments were very rare. What’s more, you can’t find this specific   pigment in nature; you have to make it.

So a vibrant purple pigment that far back   in time is an incredible achievement, but Han purple didn’t stick around.   Its usage died out with the  end of the Han dynasty,   around the early 200s CE, and it wasn’t  rediscovered until the 1990s.   When scientists first found the pigment,  they didn’t know what exactly it was.  There was no written record of how it was made,   so scientists had to rely on  experimentation and analysis to find out.   To figure out what was in the stuff,  scientists tried dissolving it in acid   as well as using techniques like bombarding  it with X-rays under a microscope,  kind of like what the Roman  concrete group did.   Thanks to these techniques, they figured  out that the purple is a compound   that contains the elements  like barium and copper  along with silicon and oxygen bound to it. They then experimented and found that   they could replicate it, or make something like it,   by cooking a barium mineral, such as witherite or barite, with quartz,   a copper mineral, and a lead salt. But it was a complicated technical feat …they needed scalding temperatures of around 1000 degrees Celsius,   and had to keep up that heat for a long time.

This all raises the question,   how the heck did the ancient scientists discover something this complicated?   One suggestion is that it was the result  of trade and cultural exchange,  since it’s similar to another ancient  synthetic pigment called Egyptian Blue,  but the logistics here seem iffy. Egypt and China are not very close to   each other geographically. Another option is that it   was discovered by glass making or trying to create imitation jade.   But why’d it stop being made?

To be honest, we don’t know for sure.   Some researchers have suggested that,  because it would have been associated   with royalty, the answer may lie in the  political upheaval that spelled the end   of the Han Dynasty and the beginning  of the Three Kingdoms period.   Others have suggested that,  since it may have been related   to the kinds of work Taoists were  doing making imitation jade,  that when Taoism became less politically  important, the work kind of stopped.   Or it might be for a completely  different reason altogether.   Sometimes we don’t know for  sure why things disappear,  making not just the artifact, but the reason  it disappeared, part of the mystery.  So, in the end, there’s a lot of stuff  that we might have forgotten how to do,  like how a particular group  of people brewed beer  or carved musical instruments, for instance.   And a lot of that we’ll  probably never recover.   But, over the past four examples, we’ve  seen how careful analysis of historical  records and samples, along with  a bit of experimentation,  can help us understand not just what past  people were doing, but how and why.   And by considering the question of  why certain techniques were lost   in the first place, we can see how  different factors can affect technology.   One way to keep from losing techniques is  to keep notes and save them in a file.   And if you’re worried about someone gaining  access to your proprietary technique,   the sponsor of this video,  Linode, has you covered.  With Linode, you’re the only person with  the keys to gain administrator access   to your server, which means you can trust  them with your company’s private data.   And they believe in open-source  and open-cloud transparency.   So not only will they keep your secrets safe, but they’re not keeping secrets   from you either. Their cloud computing can help   you store data online, host websites, create an online business, and more.  Just check out the link in the description  or head to linode.com/scishow.   That link gives you a $100 60-day  credit on a new Linode account.   Thank you for watching this episode of SciShow, and thank you to Linode for sponsoring this video. [♪ OUTRO]