Found Objects - The Stone Age Flashcards
What are stone tools made from?
Materials that can be fractures / snapped, commonly flint or chert
Can be any stone capable of being chipped into shape
What was the preferred material for making tools?
Obsidian, metamorphic form of volcanic glass, not available in UK e.g. Melos
What is flint and where is it found
Chert
In sedimentary rocks - chalk / limestone (CaCO3)
Flint is essentially quarts, consists of Si-O tetrahedra, SiO2
How is SiO2 said to be formed
- Chalk found by deposition of dead organisms, sedimented into sea 75 mill yrs ago
- in top 5m, layer of biogenic silica produced by sponges, diatoms… precipitated out at an toxic/ anoxic boundary 10m below surface
- as flint nodules sank through sea floor became purer and contained less defects
Flint knapping
Flint is chipped (knapper) into shape, any defects might interfere with the direction of fracture passing through the material
Where was the best flint for snapping found?
At the base of the flint layer, mine further for better quality flint e.g. neolithic flint mine, Grimes Graves Norfolk, deepest pits 14m
area has 433 sunken shafts
What is a crystalline material?
Has a grain structure
What is amorphous
glassy, no grain structure, and no long-range order e.g. obsidian
What is ‘cryptocrystalline’ and what materials are described as such
no grains, sheer planes e.g. flint and chert
means material will fracture in a manner directed by the application of a force of a blow –> conchoidal fracture
What happens when a blow is applied to a rock
Shock wave is generated and passes through the material gradually dies away.
Shock wave is sinusoidal, 1st peak is the largest
If the forces are sufficient the flint will fracture following the sinusoidal shape, describe the shape
- !st peak of shockwave occurs immediately below point of blow (platform)
- produces the largest ripple-shaped fracture in the flint –> bulb of percussion (characteristic sign of a man-made flint)
- ripples are known as ‘radial ridges’
How are more delicate tools made
using ‘soft hammers’ e.g. dear antlers, to transmit blow to the flint
or pressure flaking, application of a point to the flint with increasing pressure until flint failed and fractured
Describe a further method of improving the flint
Heat it to 300- 400oC for several hours, improves “flakeability”
What was earlier than the extraction of metals
the processing of metals
Most metals are found in a compound e.g. FeO, some exist in pure state and can be processed straight away, give 2 examples
gold and native copper
Why can pure metals be processed straight away
relatively low mp (1064oC), inertness, won’t react with container when melted to become liquid or corrode or oxidise in local environment
ductility = beaten into sheets and wires without breaking
Describe the properties of a single crystal of native Cu
more ductile, but less strength –> variations in properties even in what seems to be a homogeneous material
Describe the properties of Polycrystalline materials
Tend to be STRONG but not so DUCTILE
How can Cu be strengthened
Ductility can be reduced by working or hammering of pure CU = WORK HARDENING
What happens during plastic deformation
Material is being stretched
Initially deforms elastically, then plastically
represented on a stress-strain curve
What happens as a metal is plastically deformed
defects are introduced into the crystal lattice = dislocations
dislocations = lines of discontinuity in the crystal lattice that are created by the application of force
Describe an edge dislocation
Where an extra half-plane of atoms is created in the lattice, common in metals, created by an applied force, causing them to move around the lattice structure
By moving round structure, allow metal to deform and change shape, as deformation proceeds dislocations are created and forced to move
dislocations interact, can’t move past each other, more force must be applied until they do
yield strength appears to get stronger = WORK HARDENING
When and where was work hardening of Cu first used
9000 BC in Middle East
What does work hardening do?
Raises yield strength
What is annealing?
Material can be heated and allowed to cool down very slowly
What effect does heating have on work hardening?
Increases the rate of diffusion, hence increases mobility of the dislocations
becoming more mobile they can become shorter and climb up into the grain boundary, or move through crystal structure until dislocations of opposite sign meet and annihilate
Name the 3 forms of pure copper (attractive, found minerals were worked and polished for decorative purposes)
- copper sulphides
- copper carbonates
- copper oxide
Name the two types of copper sulfides
Chalcopyrite and Chalcocite (lustrous grey)
Name the 2 copper carbonates
azurite (deep blue)
malachite (green)
Name the copper oxide
cuprite (deep red)
When was the first metal extracted from its ore
Copper in 5th - 6th millennia BC by Vinca Culture of Danube, in modern Serbia
What began the Chalcolithic period
Use of copper grew out the needs of the Neolithic people, and use combined with flints resulted in the period being called the Chalcolithic
How can Cu be obtained from the ore?
By heating up the ore, resulting in dissociation from the compound in which It was found, leaving behind pure Cu
Extraction of Cup done by heating ore in the presence of charcoal
What does the Ellingham diagram show
The readiness of elements to bond to oxygen
What are the x and y axis on an ellingham diagram
X axis = Temperature
Y axis = Change in Gibbs Free Energy
What is the common line for elements on the Ellingham diagram
Most element oxides have line running upwards from left to right
Indicates the element becomes less strongly bonded to O as T increases
How does the line for CO on an Ellingham diagram run?
runs in opposite direction to other elements.
As T increases C is more strongly bound to O, CO line passes over lines for other oxides
C has greater affinity for O than Oxides lines passed over
What can C reduce
C can reduce CuO to pure Cu, for working and alloying at apps 100oC, above this temperature reduction of CuO is faster
