when talking
about ancient building techniques you
often see comments like
how are you going to chisel limestone
with a copper tool
the hardness of limestone is four or ha
ha ha how are you going to chisel
granite with a still hand chisel
when the hardness of granite is eight
i'm not making this up guys these are
real life comments
so let's try to work this out
limestone is composed of calcite and the
hardness of calcite is not
four but three
going by the mark scale
if we're discussing the mineral
composition of granite it is around
sixty percent
filled sparse thirty percent and 10
other minerals
the hardness of quartz on the max scale
is seven fields part
six so average hardness should be about
six point five not eight
come on people know your basics but a
stone's hardness isn't the best criteria
to go by
let me explain why hardness is commonly
measured on the mark scales
invented his scale over 200 years ago by
arranging common minerals with varied
hardness on a scale from 1 to 10.
it goes from talc to gypsum to calcite
to fluorite to appetite to orthoclase
to quartz to topaz to corundum to
diamond
from softest to hardest to determine the
hardness take the two minerals you're
trying to test and try to scratch one
with the other
the one that can be scratched is softer
if both are equally hard they will
scratch each other but not well
by the way
geologists don't carry the mock scale
around with them
they have two types of handy tools their
fingernails and a knife or a piece of
glass
the hardness of a fingernail is 2.5
glass or knife are roughly 5.5
anything harder than that will scratch
the glass it's a fairly simple process
just carefully scratch your samples
of course there are more precise methods
for example the brunel hardness scale
which uses a diamond pyramid to
precision mark the test sample
the hardness is determined by the depth
of the groove and the applied load
but back to our stones don't confuse
stones with minerals
a mineral is a type of a solid substance
with a specific crystalline structure
and a chemical composition
a stone can incorporate minerals
typically a stone includes a number of
various minerals
the mineral grains tend to clump
together
it's made up of grains of sand with the
hardness of seven
but does that mean that the sandstone
cannot be scored with a knife
not at all if the sandstone in question
isn't made up of solid cement but of
grains
cemented together at the point of
contact it absolutely can
all we have to do is scrape the grains
out i'll knife might chip
but so will the stone and the stone
hardness
alone isn't a good indicator
especially since people often confuse
stone hardness with mechanical
durability
they seem to think that if a chisel or
hammer is softer than the stone it will
not be able to scrape it
if that were the case geological picks
would have to be made of corundum or of
diamonds
but they are still
there's a good example of this
misconception in a quote attributed to
pliny the elder
he makes the following claim if a
diamond the hardest of all minerals is
placed onto an anvil and hit with a
hammer
the hammer may shatter even the anvil
may shift
or crack but the diamond will remain
intact
but that's not the case
a diamond can be easily crushed yes it's
hard but it's brittle
and shatters to smithereens under impact
that's a fact
there are additional parameters such as
compression and tensile strength
and shear strength these parameters are
used to describe stones
they determine experimentally and
usually are included in the stone's
documentation
but the drillability depends on the
above factors shear strength compression
and tensile strength
and the hardness of the stones it also
depends on how we go about the drilling
there's the abrasive sharp rotary
drilling method
there's the impact method there's also
the combined shock rotational
the drillability categories are defined
for each type of stone and the specific
type of instrument and method used
some stones may be easy to cut but are
hard to chip for example
jade
others are easily chipped but are hard
to cut
and minerals like diamonds can only be
cut using a diamond
on the other hand it chips easily
something the jewelers have known for
centuries
so it turns out that a stone can be
processed in different ways
for me if we're talking about impact
resistance
a stone's dynamic strength then the
hardness of the stone isn't the most
important factor
all right here we are at a quarry
near the village district
used to mine dollamentized limestone of
middle
east
[Music]
typical limestone is composed of
carbonate calcium
calcium is three on the mark scale
and the dolomite is a double calcium and
magnesium carbonate
they occupy the same position in the
crystal structure
the dolomite is the harder mineral it
has the hardness of about 3.5
to 4. in our case
by pure accident both the rock and the
mineral have the same name
anyhow one could argue that since the
copper chisel has the hardness of three
it's softer than the dolomite
limestone i should have brought some
hydrochloric acid to wipe it off
but you can see anyway that this is not
pure limestone the stone seems softer
here so let's try here
you can see it breaks up easily
and here
is and here the rock formation is denser
more solid so i'll try this
see what am i an idiot to hammer at the
hardest spot
i'm gonna try to chip at it
that seems to be the way to go
chipping at it by bit
on and on
see it goes right in and right out
hey guys do you want me to take this
whole mountain down
the bottom line is even though copper
isn't the perfect material
it can chip dolomite
and dolomite is harder to work with than
normal limestone
the egyptians didn't have birch sticks
is this better are you happy now or are
you gonna tell me that the egyptians had
no dolomite
[Music]
you