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|Some key points about flint:-
occurs as lumps in chalk beds, in Europe and North America
a form of quartz that is coloured by the inclusion of other
nodules from the chalk are coated with a thick whitish cortex
has a cryptocrystalline
structure - crystals so small they can't be seen in a
struck, flint fractures like glass
is one of the hardest materials, close behind diamond
takes a razor sharp edge, so is ideal for tools and weapons
implements tend to weather in the ground – they can go
white, brown or glossy
Flint (chert) is a form of quartz, or
silicon dioxide, also called silica. It occurs in layers and irregular
nodules in the chalk and some other limestones. It is widely distributed
around the world and was a primary material for stone age tools and
weapons. Chemically, flint is complex. It is a fine mosaic
of colloidal silica (opal) and crypto-crystalline silica (chalcedony).
|Chalk beds were formed on the floor
of ancient seas in the Cretaceous period between 145 million and
65 million years ago. Chalk is made up of individual crystals of
calcium carbonate, from the bodies of microscopically small sea
creature called Coccolithophores.
These formed a whitish mud which later solidified into
limestone rock. Flint
occurs both as layers and as irregular nodules in these chalk
beds. There are extensive chalk deposits in the United Kingdom,
France, Denmark, Poland, Russia, Ukraine, United States, Canada,
and elsewhere. All contain flint.
||A coccolithophore. (The white bar just visible
at the bottom left is 1 millionth of a metre long.) Chalk is
formed of plates from its body.
Details of how the flint formed are still uncertain.
The silica material is probably derived from the
siliceous spicules of sponges and microscopic siliceous
plankton, which are slightly soluble in water, so some of it
dissolved in the ocean and was later precipitated out onto the
chalk sea bed. Often
it collected around some solid object such as a sponge or coral,
and these are sometimes visible inside the flint when a nodule
is broken open. It’s thought that initially, the
silica must have gone through a gel-like phase before hardening
into flint, because it is found completely filling shells such
as those of sea urchins. Where
flint nodules from the chalk have accumulated in superficial deposits, such as
clay-with-flints, it is common to find internal casts in flint
of the shells that were once filled. Flint
nodules also formed in the cavities left in the sea floor by
burrowing marine animals, hence their shapes.
|A flint nodule weathering out of chalk
beds near Dover.
|Flint nodules in the chalk changed over time as their
outside acquired a whitish-coloured cortex or rind, possibly
through ground water percolating through the chalk.
It’s thought the cortex formed because the flint
surface is slightly more soluble in some microscopically small
areas than in adjacent areas, so there is differential
dissolving at the surface which results in a microscopic
sponge-like structure. The cortex can be 5mm or more in
thickness. When people started making tools from flint, they
often removed the external cortex to get at the fresh flint
inside, although in some cases they left a small
amount of cortex to make some tools, such as scrapers and
knives, easier to handle.
||Internal flint cast of a sea urchin.
|Various minerals can be included within the
microscopically small cryptocrystalline structure of flint.
Different mineral inclusions give rise to different colours.
Freshly broken Flint is commonly black, grey, green, white or
brown. The following semi-precious stones are all forms of
quartz similar in structure to flint: Agate, Carnelian,
Chalcedony, Jasper, Obsidian, Onyx and Opal.
The microscopic structure of flint is capable of holding
water molecules – as much as 30 per cent of the mass can be
water, though it is typically 10 per cent. Some or all of this
water may evaporate and leave the surface causing changes in
colour and making the flint more brittle.
Opal, for example, has a water content as high as 20 per
cent, and often loses this water, and its rainbow colouration
|Left: different coloured flint implements from China and
(right) Opal in its natural state.
On the international hardness scale, flint
ranks 7 out of 10, where diamond is 10, so it is harder than most
materials commonly encountered in the natural environment.
It also has the property of taking an edge thinner than a steel
blade (only a few molecules thick) so it is literally sharper than a
razor. Flint is still in use today as surgical tool because incisions
made with a flint blade heal more quickly and are more sterile.
Flint does not have a regular crystal structure,
like diamond, that would enable it to be cut into regular shapes like
gemstones. Because of its
cryptocrystalline structure, it shatters in a conchoidal or cone-shaped
fracture, like glass. This
property has a number of implications for someone wishing to make flint
When you strike a flint core with a hammerstone, you will always create
a fracture cone spreading out from the point of impact. If you hit near
the edge of the core, and at the right angle, the energy from your blow
will strike off a flake. If
you hit too far from the edge, or at the wrong angle, the energy from
your strike will simply create a fracture cone inside the core. One or
two mis-hits like that and your core becomes an unusable mass of
intersecting fracture cones. Flint
is thus very unforgiving to the inexperienced user. A second implication
is that one cannot cut or carve flint at will as one can with most
softer rocks – it can only be struck into flakes or blades in a
specific chain of operations that makes use of its natural method of
When a tool was discarded after use in
prehistoric times, a freshly knapped tool in many cases began to
weather, acquiring a patina over its external surface.
This patination can be affected by up to four distinct and
separate processes. First, there is the differential solution of
microscopic areas referred to earlier. The acid in rainwater will
dissolve the more soluble opaline silica from the surafce, leaving a
porous mesh of minute crystals that scatter light, creating a new cortex
that appears first blueish than whiteish in colour. This grows in
thickness over time and can be a millimetre or more in some cases.
Usually, however, patination this thick is seen only in
implements from the Palaeolithic, tens of thousands of years old. in
most flint implements from the Mesolithic and Neolithic periods, the
patina (if any) is superficial only.
The British Museum's 1950 handbook on Flint Implements refers to an
experiment in which flint was continuously submerged in rainwater
charged with the products of decaying vegetation in the presence of
chalk. Twenty-two months were needed to produce a patina 0.01mm thick.
Stages in colour-change of patination of flints
as a cortex develops: Black when freshly broken, later acquiring a
thicker and lighter patina through weathering. The flint flake shown far
right is from chalky soil.
Where flints are buried in contact with the
chalk, or with chalky soil, they tend to develop a white patina as
in the flake shown above right. Third, where the implement lodges in
ground containing iron minerals, the whitish patina absorbs the reddish
or brownish ferrous material. And fourth, there can be an additional
complex process in which molecules of quartz are dissolved from the
surface and redeposited again forming a glossy coating of opal over the
This glossy coating or sheen is different from the gloss caused by wind
abrasion in desert environments (desert gloss) and gloss caused by use
wear (sickle gloss). There is a further patination feature associated
with some flint tools and that is the occurrence of brownish spots
mainly on the ridges between flake scars. This is known as ‘iron
mould’, because its appearance resembles the spreading of a
microorganism like mould.
|An iron-stained flint
It is common to find flints, both in worked
form and as natural nodules, that have been calcined or heated by fire.
Heating changes the appearance of flint to a grey, sometimes
glossy surface, often cracked like the glaze on old pottery.
In many cases, these calcined flints are associated with camp
sites, and hence are believed to be hearth stones or ‘pot boilers’
– that is stones that have been heated in the fire and then placed in
a container of water to boil it. Some experimental archaeologists think
that people may have heated flint to some extent to make it easier to
work, though this question is still debated.
|Calcined flints or hearth stones, also
||known as 'pot boilers'