My lovely wife with the Ph. D. in metallurgy is getting golf ball sized bumps from banging her forehead on the desk over this density issue.
Don’t do that, the only time golf ball sized lumps on your head are
of any value is if you go to a SciFi convention as a Kilngon 
CIA
Perhaps the answer lies in phrasing. Instead of saying that the "material" is denser, we might say the the "object" is denser. The material doesn't change but the object made of it would vary from cast to forged..
The concept that a casting is some kind of swiss cheese in
comparison to a piece of wrought a little off. Yes you can have
porosity in a cast piece but unless it is a crap casting there is
just not that much void volume in the casting. The major problem is
the difference in microstructure, cast microstructure is just not as
strong due to grain size and shape and lack of homogeneity. So the
wrought material is normally stronger and harder but minuscule
differences in volume density from porosity are just not significant
to the discussion.
James Binnion
James Binnion Metal Arts
The concept that a casting is some kind of swiss cheese in comparison to a piece of wrought a little off. Yes you can have porosity in a cast piece but unless it is a crap casting there is just not that much void volume in the casting.
Yeah the Swiss cheese example is a little extreme. I am yet to see
any visible porosity in the casting I get from the casting house I
use. Put a nice polish on the pieces, Bob’s your uncle.
Regards Charles A.
Hi
profuse apologies over my mistake re: density vs hardness of metal.
I just thought if you squashed something it became denser.
Live and learn
Richard
Firstly I would not recommend anyone doing physical work to wear a
ring. However! I had a commission, through a shop, to make a
Titanium signet ring, it had to be bulky and have the letter M in a
heavy serifed type face standing in relief within the rectangular
sunk textured face of the ring, which was to be coloured a dark blue.
After facing off a piece of 32mm titanium rod and marking the four
quarters, I drew the basic shape I would require and marking the
centre of the finger size, for the asymmetric centre I inserted the
titanium in my four jaw chuck. Setting the marked centre as the
running centre. I drilled and turned out the finger size. Then by
carefully manipulating metal in the jaws I made a series of cuts,
about 5 per side and one for the top. This gave me the basic shape
of the ring with 12 curved facets. About 15mm deep from my original
surface.
Using a hacksaw I cut off an angle from the base of the ring to the
top then cut the ring off the rod with the oposite angle cut.
I ground off all the unwanted material using some old lapidary
grindstones under flowing water. I then had to engrave out the
background of the ‘M’ using a friend’s air engraver.
I negotiated a very good price, the whole job had to be done and
delivered in two days for an American customer.
It was great fun cutting the shape on the lathe!
David Cruickshank (Australia)
The major problem is the difference in microstructure, cast microstructure is just not as strong due to grain size and shape and lack of homogeneity
Think of the grain as rocks as an analogy. You get yourself akilo of
quartz rocks and imagine that magically they will have a magnetic
attraction to each other. You pile them together and there are huge
voids between them because they are just random rocks. Try to break
your pile and you can do it easily because the attraction varies
widely between your rocks. Now, take your rocks and pulverise them
into sand - say with a rolling mill. Now the parts are uniform and
small and they pack together tightly, with almost no voids. It’s
still quartzand it’s still a kilo, it’s that the grain structure has
been changed dramatically. Try to break that, and you’ll have a hard
time because it has become almost solid.
Now, that’s just an analogy for a clear vision and you can’t take it
too far. Jo-Ann was reading this and said “I think of it more like
wood”, which is accurate. It’s complicated, but it’s not about
density, it’s about crystalline structure - what they call “grain”
in metallurgy.
if you decrease the volume and the mass stays the same, the density
does increase.
john
profuse apologies over my mistake re: density vs hardness of metal. I just thought if you squashed something it became denser.
In theory if you could squash something so that it takes up less
volume then it would become a dense material. If metal were more like
water or a gas then you could compress it and make it more dense.
Gases and liquids are very different to solids.
The logical progression of this line of thought would be that you
could compress molten metal, and yes you can do that, but no more
than the original volume of the solid metal. When you melt metal it
expands, and contracts when it cools. It’s not only a pointless thing
to do it’s extremely dangerous.
Regards Charles A.
Charles, I don’t believe it is possible to compress water, or any
other fluid, at normal atmospheric pressures and temps. Otherwise we
would not be able to use hydraulic systems.
Also, with regards to the question of how to enable a customer to
reoxidize a piece on their own, I would suggest that they go to a
garden supply store and purchase one of the sulpher containing
dormant (oil) spray compounds. A few drops of that in warm water will
do the trick.
Dennis
I just thought if you squashed something it became denser.
I think this is correct… IF it remains the same volume! Usually
when you work metal (hammer, rolling, etc.), it squooshes out
somewhere else. But if it were contained, say in a heavy iron
container, so that hammering would be forcing it into a smaller
area, then wouldn’t it become denser? Or can’t that be done? Or if
you had a thick rod, again contained in some way, and you hammer it
at one point, wouldn’t that point be denser?
Janet in Jerusalem
Hi Dennis,
I don't believe it is possible to compress water, or any other fluid, at normal atmospheric pressures and temps. Otherwise we would not be able to use hydraulic systems.
Of course you can compress water, but it takes a lot (with a really
strong emphasis on the word “lot”) of force to do it
You can compress anything, it’s just a matter of the force required.
The compression isn’t permanent though, once the force is removed
the material goes back to its uncompressed state.
Regards Charles A.
But if it were contained, say in a heavy iron container, so that hammering would be forcing it into a smaller area, then wouldn't it become denser?
Because the atoms in liquids and solids are tightly packed it is very
difficult to compress them. It takes phenomenal amounts of pressure
to do so and there is just not much space that is available to pack
the atoms closer to each other. Once the pressure is released the
solid or liquid will return to its original density at a given
temperature and atmospheric pressure.
James Binnion
James Binnion Metal Arts
The force required to compact a piece of sterling would be far beyond
what we could do with a hammer, and once the pressure/force was
removed the metal would go back to its uncompressed state.
Theoretically, if you had a machine that was able to contain your
metal, and able to apply enough force to compress the metal, then in
theory it would be denser, but only while the metal was contained.
Once uncontained the metal would go back to its original
uncompressed volume.
Regards Charles A.
But if it were contained, say in a heavy iron container, so that hammering would be forcing it into a smaller area, then wouldn't it become denser? Or can't that be done? Or if you had a thick rod, again contained in some way, and you hammer it at one point, wouldn't that point be denser?
No doubt Jim, the wizard, will post to this, too. Not just Janet but
much of this thread misunderstands something fundamental, which
isthe very nature of matter. I won’t get very technical largely
because I forget a lot of that stuff, in detail. Look at it from the
other perspective - that of atoms and molecules, which make up all
matter. They are subject to “the four forces” which is where we
won’t go because that’s a whole physics lesson. Suffice it to say
that atoms are attracted to each other but only in certain ways and
only so much. They do that because of the electromagnetic force on a
molecular level. What that means to this thread is that atoms - we
don’t need to say molecules because all metals are “monatomic” - an
atom and a molecule are the same thing in the case of metals - will
naturally pack together, that close, no closer, no farther,
according to the attraction of the EM force. The only thing that
will change that for a given material is temperature.
What that means is that all matter is un-compressable by normal,
physical means. That means solids and liquids. In gasses the atoms
are far apart and they CAN be pushed closer together. When it is
compressed to the point where the atoms come as close as they can
come, the compression will stop and the gas will liquify. Liquids
are not compressable - as another said, that’s the root fact of
hydraulics.
So, what one is saying when the thought arises that “metals can be
compressed and the density will change” is that one can overcome the
fundamentalEM forces between atoms with a hammer. That is just not
ever going to happen, not even a little bit. Those atoms are
arranged in their natural order and nothing purely physical will
change that. In metals (and other substances) the atoms arrange
themselves in symmetrical, crystalline structures, which is the
grain that Jim has been talking about all along. But those atoms
still are only so near, and so far apart andthat is a fundamental
fact of particle physics.
Now, sure, if you take all of this to the atomic level and the
universe and all, then this can be changed. Nuclear reactors,
hydrogen bombs, the center of the sun, black holes. Under those
conditions matter becomes plasma, which is anentirely different
animal and discussion. But hitting a rod with a hammer? Not ever,
not even a little bit.
The material is in it’s natural atomic state and you just can’t
change that, that way.
It takes phenomenal amounts of pressure to do so and there is just not much space that is available to pack the atoms closer to each other. Oncethe pressure is released the solid or liquid will return to its originaldensity at a given temperature and atmospheric pressure.
I’m going to continue this because it’s important to understand and
it’s also interesting, at least to me. It’s all about energy, you
see…
Get yourself a tray and put it on a vibrating machine of
considerable power.
Fill the tray with ball bearings about 2mm in diameter, or so. At
this point, the bearings are just sitting there and there is no
energy- at least theoretically that is absolute zero. Push down on
them and nothing will happen, because they are packed tight (we’ll
disregard settling because it’s just an analogy). Turn on the
machine and the bearings will begin to vibrate, just as atoms do.
Turn the power up and up and you’ll reach a point where the bearings
have a bit of space between them due to the vibration. That would be
roughly the state of matter in the everyday world. Push on them and
you may depress something for a moment but they will just bounce
back into place - the vibration is much more powerful than any
pressure you can apply. That is STP - “Standard Temperature and
Pressure”, which is defined arbitrarily as 20C and 1 atmosphere of
pressure. That is the point at which density is measured - how much
volume the bearings occupy for how much they weigh. Turn down the
machine, the vibration lessens, the volume drops and therefore the
density rises. Turn up the machine, the vibration grows, as does the
volume (the space between the bearings is farther), etc. At any
point you can push down on your rig, but you will never (practically
speaking) overcome the vibrations. If you do, they balls willjust
pop back when you release the pressure.
Now, turn the pressure up high, and your bearings will start to flow
inside your tray. You have caused a “phase change” - there is enough
energy that the vibratingmass has become fluid. It has “melted” and
become a (virtual) liquid. You still can’t apply pressure and
accomplish anything, though. Turn it up even more and eventually
you’ll reach the next phase change. The bearings will start flying
out of the tray, and in doing that they will no longer have any real
relation to each other. That is gas, and you CAN gather the bearings
back together and press them back in a bundle. That’s why you CAN
compress a gas.
There are two factors that determine the densiy: One is above -
energy, which we usually call temperature. The other is what the
balls are made of. Styrofoam balls will have a different density
than steel balls, inherently. All of this means something, but it’s
useful to remember that we live onthe planet Earth. We are
accustomed to having our matter in a state that exists HERE, but
that’s not a universal thing by any means. If we lived on Mercury,
water would be a gas, lead and tin would be liquids and that would
be normal to us. Making ice would be akin to making liquid nitrogen,
on Earth. On Ganymede it would be the opposite - water would be a
solid, as would carbon dioxide and mercury. There would be lakes of
liquid methane that you could skate on in the winter, and THAT would
be normal to us. It’s all about energy, you see…