True or False: Charcoal is a principle component of gun powder?

True.  Gunpowder also known as black powder is a mixture of charcoal, sulfur and potassium nitrate (saltpeter).  The charcoal and sulfur act as fuels and the saltpeter acts as an oxidizer. Sugar could also be used as the fuel instead of charcoal. A traditional ratio of the ingredients would be about 15:3:2 potassium nitrate, charcoal, and sulfur by weight.

A word on how it's made:

“The second kind of flying fire is made in this way. Take 1lb of native sulfur, 2lb of linden or willow charcoal, 6lb of saltpeter, which three thing are very finely powdered on a marble slab. Then put as much powder as desired into a case to make a flying fire or thunder. Note:  The case for flying fire should be narrow and long and filled with well-pressed powder. The case for making thunder should be short and thick and half-filled with the said powder and at each end strongly bound with iron wire.” –Marcus Graecus. Liber Ignuim, ca. 1280 AD

Not all of the recipes in the Liber Ignium can be trusted but this one is pretty much dead on.



Partington, James Riddick. A History of Greek Fire and Gunpowder, The Johns Hopkins University Press, 1998

Dunn, Kevin M., Caveman Chemistry, Universal Publishers, 2003 ISBN 1-58112-566-6



True or False: You make charcoal by heating organic matter such as wood in an oxygen poor environment?

Charcoal Pit
Charcoal pit. Image credit: sarahemcc
Charcoal Pit

True. Charcoal can be made from anything containing carbon. Traditionally wood has been the raw material used to make charcoal. Wood consists of three main components: cellulose, lignin and water.  These compounds are composed almost entirely from atoms of hydrogen, oxygen and carbon.  Charcoal is made by removing the hydrogen and oxygen in the wood while leaving just the carbon.

Making charcoal consists of 4 steps.

1.)    Drying the wood to be made into charcoal.

2.)    Heating the wood in an oxygen limited environment.   Limiting the oxygen keeps the process from turning into full combustion which would reduce the wood  to ash. As the wood heats the following changes occur:

  • At 100°C the chemical bonds begin to break.
  • 100° to 200°C, noncombustible products, such as carbon dioxide, traces of organic compounds and water vapor, are produced.
  • Above 200°C the celluloses break down, producing tars and flammable volatiles.  If these are mixed with air and heated to the ignition temperature, combustion reactions occur.  That is why it is important to keep the environment oxygen poor.
  • Above 200°C the lignin in the wood starts to breakdown in an exothermic reaction. This releases additional energy which can cause the temperature of the wood to rise to 400°C or more. 

3.)    The wood should continue to be heated to between 450- 500°C.  A temperature of 500°C gives a typical fixed carbon content of about 85% and a volatile content of about 10%. The yield of charcoal at this temperature is about 33% of the weight of the oven dry wood. 

4.)    The wood is allowed to cool in an oxygen limited environment to prevent the oxidation (combustion) of the remaining carbon.


Charcoal pit sketch by Nancy Aldrich
Charcoal Pit

Traditionally this was done by piling dry wood into a dome shaped mound.   The mound was then covered with smaller branches, leaves and finally dirt.  Covering the mound limited its exposure to oxygen. A flue was left open in the middle of the mound to introduce hot coals and start the mound to smoldering.  The shape of the mound is important because as the wood transforms to charcoal it shrinks in size.   This shrinking inevitably causes holes in the outer covering of dirt which allows more oxygen into the mound.  These holes have to be plugged quickly or the mound will catch fire and all the potential charcoal will go up in smoke.  Planning for the shrinking of the mound helps to minimize the work in plugging holes as they appear.   The key is to keep the entire mound smoldering but not burning.  This requires constant attention.  If there isn’t enough oxygen the mound will cool too much and if there is too much oxygen the mound will catch fire.  The Swiss have been making charcoal this way since the Middle Ages. It might take two men three weeks to build the mound of 700kg (1500lbs) which will then burn for 12-18 days.  The charcoal burner must spend the entire time by the mound tending to it as it smolders.


Charcoal Barrel
Charcoal barrel

A more modern method (and one that requires less attention) is to seal up the wood in a fireproof container with a small hole in it so that it can vent the gasses that are produced and then place the container in a fire or kiln.   This allows more attention to be paid to the fire that is providing the heat without having to worry that the wood being charred is being exposed to too much oxygen.  Here a steel barrel has been converted to a charcoal kiln.  The wood would be loaded into the barrel and then it would be sealed up.  A fire would be lit underneath the barrel to provide the necessary heat.  Note how it vents the gasses from the barrel back into the fire with the tube that comes out of the barrel.  This set up could be made much more efficient if the barrel were surrounded by earth or bricks to help trap the heat.   (Photo from


Odd Notes: The process of driving off the hydrogen and oxygen by thermal decomposition is called pyrolysis.  Pyrolysis is the same process that is used to turn coal into coke. It is also used to make carbon fiber. When carried to an extreme so that it leaves mostly carbon residue it is called carbonization.



What secret would have allowed the ancient Egyptians to create hydraulic cement (like Portland cement)?

Clinker kiln used to create Portland cement.

The Egyptians could have been building structures out of concrete if they had only known the secret to making hydraulic cement.  In the early 19th century, a bricklayer named Joseph Aspdin in Leeds, England first made Portland cement by burning powdered limestone and clay in his kitchen stove. He named it Portland cement because of its similarity to Portland stone, a type of building stone that was quarried on the Isle of Portland in Dorset, England. The secret to Portland cement are the compounds belite (Ca2SiO4) and alite (Ca3O·SiO4). When they are mixed with water (hydrated) they form crystals that grow like tiny rock-hard fingers wrapping around the sand and gravel creating concrete. This means that concrete will harden underwater. Concrete doesn’t harden because it dries out. It hardens because the Portland cement (which is a hydraulic cement) uses water to form crystals within the matrix of the concrete. The compounds responsible for this are created by heating a mixture of ground limestone and clay (or shale) to temperatures between 1400-1450 °C.


A modern manufacturing process consists of three stages:


1.) Grinding a mixture of limestone and clay or shale to make a fine "rawmix.

The limestone contributes calcium carbonate while the clay or shale provides the silicon and aluminum oxides needed to form belite and alite. Limestone with some impurities is preferred to limestone that is pure calcium carbonate.


2.) Heating the rawmix to a sintering temperature of (1400–1450 °C)

This fuses the rawmix into lumps or nodules which is called “clinker”. The clinker once cooled is relatively stable and can be stored.


3.) Grinding the resulting clinker to make cement

The grinding is usually done with gypsum to facilitate the grinding and to prevent flash setting (premature loss of workability or plasticity of cement paste) of the cement


Grinding the limestone to a fine enough powder before mixing it with clay would be difficult with the grinding technology of the Ancient Egypt so it would be better to comminute the limestone by burning and slaking than by grinding. This would be done by heating the limestone to 900°C for several hours which would turn the limestone into quick lime. After it has cooled, the quicklime would be combined with water to form slaked lime.  This slaked lime would be combined with the clay or shale to form the raw mix and then steps 2 and 3 would be resumed.

In the second stage as the rawmix is heated, these chemical reactions take place as the temperature of the rawmix rises:

  •  70 to 110 °C – Free water is evaporated.
  • 400 to 600 °C – clay-like minerals are decomposed into their constituent oxides; principally SiO2 and Al2O3Dolomite (CaMg(CO3)2) decomposes to calcium carbonate, MgO and CO2.
  • 650 to 900 °C – calcium carbonate reacts with SiO2 to form belite (Ca2SiO4).
  • 900 to 1050 °C – the remaining calcium carbonate decomposes to calcium oxide and CO2.
  • 1300 to 1450 °C – partial (20–30%) melting takes place, and belite reacts with calcium oxide to form alite (Ca3O·SiO4).

Alite is the characteristic constituent of Portland cement. Typically, a peak temperature of 1400–1450 °C is required to complete the reaction. The partial melting causes the material to aggregate into lumps or nodules, typically of diameter 1–10 mm. This is called clinker.

The clinker once ground with gypsum is a hydraulic cement than can then be used to create concrete. Below is the depiction of a modern rotary kiln.  Ground limestone and clay enter as raw materials and clinker falls out the other end.

Clinker kiln used to create Portland cement.

Image from

Where do you find flint or obsidian to make stone tools?

For the sake of simplicity, we have lumped the preferred materials for making stone tools into two broad categories; flint-like and obsidian. The flint-like materials (such as chert) are mainly made from Silicon dioxide (SiO2) which is also known as silica.

Chert is created when microscopic single-celled algae die and their silica rich skeletons fall to the ocean floor and dissolve and reform. The microcrystal formation of the silicon dioxide is what gives the flint-like materials their unique properties (hardness and conchoidal fracture). Chert nodules are often found in large deposits of chalk and some of the oldest mining operations in the world are in chalk deposits where they mined for chert. The mines at Grimes Graves are a good example of man mining for flint as long as 5000 years ago. It is interesting to note that they often rejected many shallower seams of chert in favor of better quality chert.

Bottom line: When searching for flint-like stones, search for large deposits of chalk or limestone. Search for fault lines or mountain ranges that may have exposed layers of an ancient sea floor. Large rivers or river beds might hold chert nodules that washed down from their source. Dont' forget that most chert will be surrounded by a cortex of material that will not make it stand out from other stones in the area.

Obsidian can be found near old lava flows because it is essentially natural glass. It can be found where lava contacted water, or where it cooled while airborne or along the edges of a lava flow. Obsidian is susceptible to weathering and as such it is rarely older than a few million years.

You want to make a stone tool. What type of stone should you look for?

Examples of Chert
Chert formations, Pindus mountains Greece(source: candiru)
Examples of Chert

Our ancestors discovered that certain stones were very hard and, when broken, they formed very sharp edges.   These were logically used to produce cutting tools such as knife blades, arrowheads, axe heads and scrapers.  Geologists have refined classification of these various stones into categories such as flints, cherts, jaspers, chalcedonies, agates, quartz, obsidian, etc.  For the purposes of simplicity it is easy to lump them into two broad categories: Flint-like and obsidian. 

Flint-like stones all have a microcrystalline structure that breaks with a conchoidal fracture.  They are very hard (about a 7 on the Mohs Scale).  Their hardness and fracture qualities make them ideal for creating a durable cutting edge.   If you need a durable cutting tool, a flint-like stone would be your best bet.

Obsidian is natural glass that was made by volcanic action.  It also breaks in a conchoidal fracture and has a hardness of about 5.5 on the Mohs scale.   Since it was formed by quickly cooling, it does not have a crystalline structure.  Obsidian fractures to form an edge even sharper than the flint-like stones. Fractured obsidian can form an edge sharper than high-quality steel scalpels.  Since it is essentially glass it is more fragile than the flint-like stones.  If you needed to create a scalpel, then obsidian would be your best bet. That’s not to say that you can’t still make a fine tool from it.  It is just softer and more brittle than the flint-like stones. Obsidian can also be polished to form a mirror.




True or False: Concrete can be made by mixing powdered limestone, sand, gravel and water?

The Pilgrimage Church by Gottfried Bohm was opened in 1962 in Neviges Germany
Interesting use of concrete for construction  (source: seier+seier)
The Pilgrimage Church by Gottfried Bohm was opened in 1962 in Neviges Germany

Concrete is the most often used construction material of the modern world.  It is simple and economic to make and if the Egyptians had known the secret they could have made it 4000 years ago. To make concrete, first you need to make the cement (or mortar) that holds the concrete together. 

The Romans did this by burning limestone (which is mostly calcium carbonate CaCO3) to create something called quick lime (calcium oxide CaO). Burning isn't quite the right term because "to burn" usually indicates an oxidation process which involves the "chemical union of oxygen with any substance".  What actually happens in a lime kiln is that the temperature of the limestone is increased to a point above its dissociation temperature.  The earliest lime kilns were really just layers of wood and limestone stacked together and lit like a giant bonfire.  So it is easy to see why it is called burning limestone when it really isn't.

Anyway, when the limestone gets to at least 900 deg C for several hours,  it releases carbon dioxide (CO2) and leaves behind quick lime (Calcium oxide CaO).   Calcium Oxide isn’t stable and it will start pulling carbon dioxide out of the air and turning back into Calcium carbonate.  So you can’t store it long term unless you find some way to seal it in a container without carbon dioxide.   The checmical equation is shown below.

Calcium Carbonate + Heat = Calcium Oxide + Carbon Dioxide
   {CaCO3}                            =   {CaO}         +   {CO2}

Once the quick lime has cooled it is mixed with water to form Calcium Hydroxide (Ca(OH)2) which is also called slaked lime or hydrated lime.  This reaction produces quite a bit of heat and has been used to create self-warming beverage cans or heaters for meals. Depending upon how pure the initial limestone was it can produce hydraulic or non-hydraulic lime.  Hydraulic lime will set underwater (it needs water to cure) and non-hydraulic lime sets in air (it needs carbon dioxide to cure).  Impurities such as silicon oxide, aluminum oxide, ferric oxide will cause the formation of hydraulic calcium hydroxide.  The Romans introduced such impurities by adding volcanic ash or pumice to the calcium hydroxide.  The Romans mined such volcanic ash near the city of Pozzuoli, Italy and the ash came to be known as Pozzolana.  Typically it was mixed in a ratio of roughly 1:3 (hydrated lime: volcanic ash) for buildings and roughly 1:2 for underwater work.

Now that you have a hydraulic cement (or mortar) you can mix it with an aggregate such as sand or gravel and create concrete. But the Roman engineers had one more trick up their sleeve.   If you were to mix these ingredients together in the manner of modern concrete and place it in a structure, the result would certainly not hold up as well as the Roman concrete has.  That is because the Romans hand mixed the hydraulic lime and volcanic ash in a box with very little water and then transported it to the site where it was then placed and tamped (or pounded) into the voids of the rock layers.  This tamping reduced the need for excess water which is a source of voids and weakness. 

So it is false that you can make concrete with powdered limestone, sand, gravel and water.  Mixing that will just make a muddy mess.