Heat-treatment
can significantly change the physical properties of a number of types
of rock, enabling the manufacture of stone tools of a higher quality
than would otherwise be possible from the same material. The
discovery of this by early humans is considered a significant
cultural breakthrough for stone age societies, representing one of
the first known instances of the intentional transformation of
materials for technological reasons in the archaeological record.
This breakthrough appears to have been made independently on a number
of occasions around the world in the Late Pleistocene and Early
Holocene, with the earliest known occurrences being at Pinnacle Point
and Blombos Cave in South Africa, where the technology first appears
about 70 000 years ago. In Southern Africa this technology is
associated with the advanced Middle Stone Age Still Bay and Howiesons
Poort cultures, both of which are known to have made extensive use of
fire, which flourished 71 900-70 000 years ago (Still Bay Culture)
and between about 65 800 and 59 500 years ago (Howiesons Poort
Culture) respectively, then apparently disappeared and
were replaced by less advanced cultures. However while individual
stone tools with signs of heat-treatment have been identified,
establishing how widespread and systematic this approach to
tool-making was is much harder, leaving open the possibility that
some heat-treated tools may have been made opportunistically from
stones that had not been heated with the specific intention of later
making tools.
The
Klipdrift Shelter has produced a series of layers about 120 cm thick,
containing well preserved archaeological assemblages, including
marine and terrestrial faunal remains, organic materials, lithics
(stone tools), ochre and engraved ostrich eggshells, dating from
about 70 000 to about 50 000 years ago. The layer PDB layer has been
dated to 64 600 years ago (plus or minus 4200 years) and is noted for
a particularly rich lithic assemblage, with about 2500 stone tools
recovered from an exposed surface of about four square meters. These
artefacts are considered to be of the intermediate Howiesons Poort
technology phase, with a large number of notched tools and a
predominance of silcrete items. Of the knapped tools from this layer
46% were found to be made of silcrete, 35.4% of quartzite, 17.1% of
hydrothermal quartze and 1.5% of chert or calcrete.
Map of the excavated area at KDS and site stratigraphy with OSL
dates. Magnus Haaland in Delagnes et al. (2016).
Heat-treated
can be used to modify a variety of rocks for making stone tools, most
notably flint (chert) and silcrete. However the temperatures needed
for heat-treatment of flint are far higher than those needed for
heat-treatment of flint, and only heat-treated silcrete tools have
been found at Pleistocene South African Sites. Silcrete is a highly
variable rock, and the effects of heat-treatment upon it are also
variable, which makes it difficult to simply pick up a piece of
silcrete and say 'this has been heat-treated'. Typically when
heat-treated silcrete becomes more brittle, loses porosity, is
reddened and may fracture. However without knowing the original
properties of a piece of silcrete it is difficult to assess whether
it has been heat-treated.
Silcrete
is not naturally occurring at the Klipdrift Shelter, rather it
outcrops at a number of sites in the Cape Fold Mountains about 10 km
to the north of the site. In order to assess whether the silcrete
tools of layer PDB had been treated with fire, Delagnes et al.
collected a series of comparison samples from these sites. These
samples were not thought to necessarily match the outcrops which
produced the material for the stone tool production, but were chosen
for similarities in lithological properties (grain size, mineral
composition etc.).
Map
of the KDS area showing the six sampling locations for our reference
collection. The black spots correspond to primary silcrete and
ferricrete outcrops georeferenced by Roberts (2003). The red x of
source 5 indicates that this outcrop was not georeferenced by
Roberts. Gauthier Devilder in Delagnes et al.
(2016).
By
comparing silcrete artefacts from layer PDB to both heat-treated and
non heat-treated samples from this reference collection, Delagnes et
al. were able to ascertain that
92% of these objects showed signs of heat-treatment prior to
manufacture, while of the remaining items the majority were
impossible to assess (primarily due to exposure to fire after their
manufacture, probably as a result of being discarded in places where
fires were subsequently built), while only 1.5% of the items were
identified as having been manufactured without the use of fire.
Heated
blades and tools on blades from Klipdrift Shelter, layer PBD.
(1)-(16): Silcrete blades showing their diversity in terms of size
attributes and visual transformations after heating, (17)-(19):
Backed tools including one fragment of backed tool (17), one
bi-truncated tool (18), one segment (19); (20), (21): Notched blades;
(22), (23): Blades with slight continuous retouch on one lateral
edge. Delagnes et al.
(2016).
However, simple heat-treatment prior to production does not appear to
have been the entire story of the manufacture of stone tools at
Klipdrift Shelter. Around 25% of the tools show signs of having been
worked both before and after heating, suggesting that the
heat-treatment was part of an extended manufacturing process rather
than a simple pre-treament. Furthermore about 7% of the items show
signs of heat-induced fracturing, which does not appear to have been
a catastrophe to the tool-makers, since they were able to work these
incidental surfaces into their eventual design rather than being
forced to abandon the objects.
Heated
blade cores from layer PBD, Klipdrift Shelter. Caption for drawings:
(1) Knapping platform preparation, (2) convexity preparation, (3)
blade removal without initiation, (4) blade removal with initiation,
(5) indeterminate removal, (6) cortex, (7) preheating surface, (8)
heat-induced non-conchoidal fracture (HINC), (9) post-heating
removal. Note that all three cores (A, B, C) show a sequence of core
exploitation (preparation and blade production) that follows a heat
treatment which has resulted for A and C in heat-induced fractures,
and which was preceded for A and B by a first stage of core
exploitation. Delagnes et al.
(2016).
Delagnes
et al. further reason
that it is likely that a higher proportion of blocks underwent
heat-induced fracturing during preparation, as in many cases
reworking may have subsequently removed all traces of the heat
fractured surface. With this in mind they reason that the deliberate
heat-fracturing of material may even have been part of the
manufacturing process. Such fractures tend to occur along plains
where there are pre-existing (but invisible) weaknesses in the rock,
such as those caused by iron oxide or hydroxide concentrations (which
produce water vapour when heated, fracturing the rock), preventing
wastage and accidents later in the manufacturing process. Such
deliberate heat-fracturing of rocks has been recorded among modern
stone tool-makers in North America, the Andaman Islands and Zimbabwe,
and is believed to have been used by the tool-makers of the
Sauveterrian culture of the French Mesolithic (8500-7500 years ago).
See also...
