According to historical records, people have discovered coal from the late Spring and Autumn period to the early Warring States period, which is a black stone that can be burned, and is called Shini in the "Classic of Mountains and Seas".

The earliest records of coal mining began in the Western Han Dynasty. It is recorded in the biography of foreign relatives: Dou Guangguo, the younger brother of the Empress Dowager Dou, went into the mountains to mine coal, and a landslide accident occurred in the process of coal mining, crushing more than 100 people. Hundreds of people died in a single landslide, indicating that the scale of coal mining is already very large.

However, in the Three Kingdoms period, the application of coal was not yet popular, and people still relied on wood for heating in winter.

Of course, Cao Liang knows that Shanxi (Bingzhou) has always been known as the "sea of coal", and said unceremoniously that in the Jinzhong area, if you find a random place and dig a hole to dig down, you will dig coal nine times out of ten, and Shanxi's rich coal reserves are indeed breathtaking.

But now these black treasures are still lying quietly under people's feet, unnoticed. Nowadays, many mountains have been burned by the Huns, resulting in a large shortage of firewood, and it will be winter in a few months, and the weather in Bingzhou is much colder than Luoyang, there is not enough firewood, and the heating of hundreds of thousands of soldiers and civilians in Bingzhou is a big problem.

So Cao Liang set his eyes on the ground under his feet, the treasure hidden under the ground is a good thing, as long as some of it is picked out, there is no problem in solving the winter of the military and civilians in Bingzhou.

But coal mining is a technical work, most of the coal is buried deep in the ground tens of meters to hundreds of meters, even up to thousands of meters, need to dig a very deep mine to mine coal, and because of the threat of gas, water permeability, collapse accidents, so that even in the later generations of coal mining technology has been very developed, accidents still continue to occur, throughout history, coal mining has always been a high mortality and high risk industry.

It's much better if there is an open-pit coal mine, just dig up that layer of soil on the surface, and the rest is much simpler. But it is a pity that Shanxi's open-pit coal mine is in Pingshuo, outside Yanmen Pass, and there are still hundreds of miles away from Jinyang, not to mention that the area is still under the control of the Hu people.

It seems that if you want to dig coal, you can only find a way in the Jinzhong area, Cao Liang set his eyes on the place around the Yangbian Mountain of Daling Terrier, which was a small coal mine in this area in later generations, and there was a small coal mine in almost every ravine.

Cao Liang now has no shortage of labor at hand, and there are as many as 10,000 Xiongnu soldiers captured in the Battle of Dongguan, plus the captives of Dalingzi clan and other places in Qi County, Cao Liang can at least make up 20,000 or 30,000 young and strong laborers.

Originally, according to the plan, these people were consumed in the endless struggle, but now Cao Liang seems to have a better idea, that is, let them dig coal, to extract his surplus value to the maximum, the black coal kiln is a place to eat people, drive these 20,000 or 30,000 people to dig coal, and after a year, it would be good to have a percent of survivors.

There is no way, now that the Wei army is short of food and grass, it is impossible to feed so many Xiongnu prisoners of war, but Cao Liang is unwilling to bear the infamy of a butcher, and it is even more impossible to let them go in vain, so it can be said that it is a better way to let them dig coal.

Cao Liang ordered these Xiongnu prisoners of war to be escorted to dozens of ravines in the Daling Jiaoyang belt, each of which held hundreds to a thousand people, these ravines could be said to be natural prisons, and the Wei army only needed to send a small number of soldiers to block the valley mouth and guard the mountain roads to firmly control the situation.

When the Xiongnu prisoners of war were escorted to these ravines, they aroused inexplicable fear, believing that it was likely that the Wei people would execute them in the ravines and bury their bodies here, and the people outside knew nothing.

Panic led to a commotion, which ensued a riot, but the Wei army seemed to have been prepared for this and suppressed it harshly.

Unarmed Xiongnu prisoners of war could not have been a match for the well-armed Wei army, and after the execution of several rioters, order in the valley returned to normal.

In fact, these Huns thought too much, and after settling them down, the Wei army issued them with tools such as pickaxes, shovels, hammers, and baskets, and announced that they would dig a black stone in this ravine.

The Huns felt a little incredible, what is the use of digging black stones, is it that the Wei people think they are idle and panicked, give them something to do.

But finally able to survive, it is also a good news, after being captured, these Huns are desperate, referring to their methods of dealing with the Han people, it is definitely nine deaths, now they can eat, even if they do some work, it is nothing.

In order to prevent the Xiongnu from using these picks, shovels, hammers and chisels as weapons, so the number of these tools distributed was relatively small, and the Wei army divided the Xiongnu prisoners of war into several groups and took turns to work in the well.

And these tools are under strict control, all the tools, all left in the well, the Huns who change shifts are not allowed to take out, even if the tools are damaged, they must be handed in after the bad parts can be picked up.

After each day's work, all tools must be handed in, and none of them can be kept until the next day's work is undertaken.

The Wei army also had a special person to keep these tools in order to ensure that nothing was lost.

In Cao Liang's impression, there are small coal mines in these ravines, but Cao Liang really doesn't know anything about the specific location of each coal mine and the depth of the coal seam buried, so in the initial stage of mining, Cao Liang adopted the method of casting a wide net and fishing more, and in each ravine, several points were selected to dig and drill wells at the same time.

If he didn't find the coal seam even after digging dozens of zhang underground, then Cao Liang would choose to give up, because if he digs any further, there will be a serious lack of oxygen in the mine, and people will suffocate to death after staying in it for a long time.

Although Cao Liang was not prepared to treat these Xiongnu prisoners of war as human beings, it was undoubtedly not cost-effective to die a large number of them before they dug coal, so when Cao Liang first dug the well, the first thing he considered was the problem of ventilation at the bottom of the well.

In order to ensure ventilation at the bottom of the well, Cao Liang also designed a set of ventilation devices, a hand-cranked blower, through a long wooden pipe, can blow fresh air from the ground deep underground, so as to ensure that there is no lack of oxygen under the well.

At the same time, after digging coal in the future, the gas at the bottom of the well can also be pumped out through this ventilation device, and only the blower needs to be reversed.