Flint

Flint, also known as hornstone, chert or silex, is one of the most common, hardest and most durable rocks. Because of these characteristics it has been used as a building material since Roman times. Many Roman villas were constructed with flint walls, a tradition that continues in southern Britain until the present day.

Flint was also used for the manufacture of gunflints because of its ability to produce a spark when struck with metal, which is reflected by its Dutch name vuursteen, firestone. This characteristic was already known since prehistoric times when flint was used as a firelighter by striking it with iron containing ores such as pyrite and marcasite.

What is flint

Flint is a hard glass-like cryptocrystalline form of quartz. It is comprised of silicon dioxide (SiO2), also called silica. In principle this mineral is colourless and transparent but depending on its microcrystalline structure and the type of mineral inclusions, flint can be dark-grey, black, green, white, or brown in colour. Because of its structure, flint shatters in a curved shell-like (conchoidal) or cone-shaped fracture, just like glass. Like glass, when it fractures it has the tendency to produce splinters with a very sharp edge, which can be thinner than a steel blade and sharper than a razor. With and edge of only a few molecules thick flint blades are presently used as the cutting edge in some of the finest surgical tools. Flint ranks 7 out of 10 on Mohs hardness scale, where diamond ranks 10, so it is harder than most materials commonly encountered in the natural environment.

The sharpness of its flakes, the predictability of fracture and its hardness made flint the ideal material in prehistory to produce a multitude of tools. The use of flint for the manufacture of tools goes back more than a million years making these implements the most common remains in archeology. They are virtually indestructible and remained preserved over the enormous span of time during which homonids made them.

Origins of flint

Flints occur everywhere in the world. The oldest know flint deposits date from the Cambro-Ordivician era (35-40 million ya, New Jersery) and the youngest from the late Tertiary period (3 million ya). The best know flint deposits are of course those found in the sedimentary chalk and limestone deposits of the late Cretaceous period (60-95 million years ago). Although flint consists of inorganic silica, its origins are biogenic, formed from the remains of sea sponges and siliceous planktonic micro-organisms, such as diatoms and radiolarians.

The exact mode of its formation is still hotly debated. In tropical climates the soil can contain a lot of dissolved silica and it is thought that this could have been transported to the Cretaceous seas via rivers, where it was used by the above mentioned organisms to produce their silicous exo-skeleton. After they died the silica would have been released. Most theories favour the early precipitation of silica gel within a zone of mixing 1-2 meters below the tropical seabed where there was a chemical boundary between oxidative conditions at the top of the seabed and reducing conditions below. It is thought that this process didn’t happen at once but occured in slow steps and at different stages.

Within the mixing zone the silica gel formed layers and irregular nodular concretions. It filled up burrows and cavities and enveloped the remains of marine creatures, before dehydrating and hardening into the microscopic quartz crystals which constitute flint.

Shown on the left are Cretaceous flint deposits on the North Sea coast of Norfolk near Cromer. In the foreground a large paramoudra can be seen where flints have deposited around a large vertical burrow.

Shown here are casts of Cretaceous sea urchins left behind after silica filled the urchins' shell. The fossilised shell of the urchin on the left has since eroded away. The specimen shown on the right of the image is a partially siliconised urchin with parts of the fossilised shell still intact.

Some flint tools have been found decorated with fossils. The Neolithic scraper/knife below shows a sea urchin imprint (Rijckholt, southern Netherlands). The most enigmatic examples of these include a Lower Palaeolithic handaxe with a sea urchin cast, from Swanscombe in Kent (Liverpool Museum), and one with a bivalve shell imprint (Museum of Arch. & Anthropol., Cambridge). Surely the makers must have looked in wonder at these strange markings, but what they would have thought about them we will never know.

Sources of flint

Marine Cretaceous deposits with flints can be found all over the world and shown on this map are the many sites in Europe where flint was avalable for the stone age toolmakers (source: University of Aukland).

The colour of flint can be a helpful indicator of its source. Although flint in its natural colour is black or grey, chemical changes brought about by interactions with minerals, during its formation or in the moisture absorbed from the environment, can produce a range of colours and banding patterns.

The flintsource web site can be a useful tool to help identifying the source of the raw material from which a flint tool was made. Shown below are several examples of tools made from a typical type of flint indicative for the region from which it was extracted.

This Neolithic knife was made from dark-grey and light-grey banded Rijckholt flint (Rijckholt, southern Netherlands) a type of flint that was already in use during the Upper-Palaeolithic period but reached its peak in the Neolithic when it was extensively mined in this area.

Polished Neolithic axe head made from mottled brown Danish flint (northern Denmark).

Mesolithic awl made from Lousberg flint (Sweikhuizen, southern Netherlands). This type of flint is found north of the city of Aachen in Germany and is characterised by a core of grey flint surrounded by a zone of reddish brown flint.

This large Neolithic livre de beurre core is made from lustrous honey coloured Le Grand-Pressigny flint found in the valley of the Claise river (Loire Valley, France).

Patination

Colour changes are caused by changes in the internal structure of the flint, resulting in repeated refraction and reflection of light by the numerous microcrystaline surfaces. In many cases these changes create a chalky white patina where most of the incoming light if reflected back to the observer. The colour of the patina of a flint correlates with the type of environment it has been exposed to and its thickness is linked to the extend of the exposure. Flint implements from the Neolithic and Mesolithic, for instance, show less patination compared to those of the Palaeolithic. Attempts to correlate patina thickness with age, as a tool for dating flints, have so far proven unsatisfactory because many of these changes depend on the texture and internal microstructure of flint, its permeability, and the kind, proportion, and distribution of impurities.

Some artifacts only show partial patination. This is an example of a Middle Palaeolithic handaxe made from Le Grand-Pressigny flint showing a different patination on each side.

White patina

A white patina often results from silica dissolving from the outer layers of the flint. This causes an optical effect that is comparable to a head of foam on a pint of lager. Similar to the lager example, chemically there is no difference between the composition of the patina and the flint in the core. White patination is often found linked to alkaline soils but also acidic sandy soils and peat.

Implements can be found covered with a uniform layer of white patina. The flint tools shown here are from the La Micoque site in the Dordogne (France). They are coated in a thick layer of white patina, leaving only a small residual core of the original black flint behind, after hundreds of thousands of years of weathering.

Irregular white patina’s can be found as well, giving the flint a bluish hue, especially on black flints. Some of these can show criss-cross patterns, known as vermiculé or basket-work patina, which is thought to be caused by plant roots. Flints with a white and bluish are often associated with Chalk downland environments. This example was found near Cromer in Norfolk.

Colour patina

Yellow, brown, red and green colourations of implements are caused by mineral oxides and hydroxides that can enter flints via the absorption of moisture. Flints are very porous and and water perculating though the flint’s cryptocrystalline structure can either dissolve and remove or precipitate these minerals. Oxidation of iron containing minerals in the outer layers of the flint can lead to yellow, brown and red colour variations which are typical for river terrace gravels. Shown here is a bifacially worked tool from Midsomer Norton made from black flint with a heavy river terrace gravel patina.

Sometimes iron oxides penetrate the weaker spots of the implement, on or near ridges or edges, where they can form rust-like spots known as iron-mould, as can be seen on the flake with the urchin impression (see above).

Iron sulfur containing minerals can give flints a greenish hue which can be observed in implements that are found in anaerobic waterlogged conditions such as peats, wetlands and rivers.

The tools shown here are made from mottled flint that have acquired a glossy black and olive river patina (Lower Avon, Bath).

A black patina is also often found in deposits exposed to sea water. These black patinated flints come from the beach near Le Havre (France).

Gloss patina

Gloss patina is thought to result from deposition of silica in the interior parts of the flint after dissolving out from the outer layers. This type of patina can often be found on Upper Palaeolithic flints, rather than Middle and Lower Palaeoliths.

If a flint implement was used to process plant materials often the silica released from this material gives it a bright use-wear polish as can be seen on this flint axe head from Portesham (southern England).

Artifacts that have been exposed to the abrasive effects of wind and sand can also have a polish. The pockmarked surface of this Middle Palaeolithic artifact from Sweikhuizen (southern Netherlands) is covered in waxy gloss patina caused by wind abrasion during the periglacial conditions at the end of the Last Ice Age (also referred to as the Last Glacial Maximum or Weichselien/Devensian glaciation).

Wind abrasion and other traces of glacial transport damage, such as pressure cones and frost fractures are often found on surface finds from the Lower and Middle Palaeolithic, north of the glacial boundaries.

Further reading:

Beuker, J. Vuurstenen werktuigen: technologie op het scherp van den snede (2010). Sidestone Press, Leiden. Dutch.

Werkgroep Prehistorische Vuursteenmijnbow. De prehistorische vuursteenmijnen van Rijckholt-St. Geertruid (1998). Nederlandse Geologische Vereniging. Dutch.

Butler, C. Prehistoric flintwork (2008). Tempus Publishing

McNamara, K.J. The star-crossed stone: the secret life, myths, and history of a fascinating fossil (2011). The University of Chicago Press.