Something interesting happened to me a couple of years ago!
I was about to refine some gold scrap by inquartation, and did this by
alloying it with silver.
To have a smooth reaction, it is paramount that the metal is finely
granulated (or it will take ages to dissolve the silver and base
metal, but you all know this). To granulate it, I simply melted it, then dumped it HARD into
a steel pot with 5 cm of cold water at the bottom. Approximately 40
grams of metal, the hotter the better and its totally uneventful the
first three times, the result being a foil like metal. The fourth time
however, immidiadly upon dumping, there was a dull crack, and water
and (luckily already cooled) very finely granulated metal was
propelled explosively out of the pot. Most of the metal stayed in the
pot, but some was recovered from the wall, table, exhaust and my hair.
The granules varied in size from jagged bits 2-3 mm across to
something almost like flour.
Don't understand quite why it happened, but it is a good argument for
remembering wearing the safety goggles, even when doing something you
have done a hundred times and consider to be harmless.
Peter
(Now wearing a pair of fresh shorts!)
I personally don't believe in the "spontaneously trapped water hypothesis" (I do believe in the explosion after trying one!) Surface tension and
difference in density should eliminate that, unless the water and the
metal are forced into contact.
When molten silver is poured into water, surface tension attempts to
draw it together in a ball (or burger, thanks to gravity) and the
water will not contact it directly, thanks to the Leidenfrost effect.
A thin film of steam is formed at the surface preventing the water
from touching the metal, greatly lowering the thermal transfer.
Se http://www.fysikbasen.dk/index.php?page=Vis&id=12 for some nice
pictures, and
http://www.wiley.com/college/phy/halliday320005/pdf/leidenfrost_essay.pdf
for a thorough explanation. (dropped it in the files folder too).
If a red hot silver coin is dropped in shallow water, it sometimes
just sits at the bottom making a buzzing sound for a couple of
seconds, then suddenly cooling with a sharp, almost clicking sound.
(or a drop of water racing around on a hot stove, most people know
that effect)
This is just a theory, but if the molten metal should start to
oscillate, breaking the film of steam, or be cooled locally,
collapsing the film in that way, the evolution of steam could push on
the metal, propelling it through the steam causing more explosive
steam formation, and the reaction would run wild. A local fast
formation of steam would certainly raise the pressure sufficiently to
powderize the metal and accelerate some water out of the container
(the energy is present, its just a question of releasing it fast enough)
http://www.ganoksin.com/orchid/archive/200404/msg00558.htm
"
Certainly sounds like oscillation to me.
I superheated the metal, end dumped it as hard as possible, which
could have forced it to spread and contact the water (that was in fact
what I intended, I certainly got what I wished for!).
If we can get a clearer picture, we might reach a testable hypothesis
(I like blowing things up, but prefer that it happens when I intend it
to! And cheaper things may make a better boom if thats the purpose!)
Peter
I was about to refine some gold scrap by inquartation, and did this by
alloying it with silver.
To have a smooth reaction, it is paramount that the metal is finely
granulated (or it will take ages to dissolve the silver and base
metal, but you all know this). To granulate it, I simply melted it, then dumped it HARD into
a steel pot with 5 cm of cold water at the bottom. Approximately 40
grams of metal, the hotter the better and its totally uneventful the
first three times, the result being a foil like metal. The fourth time
however, immidiadly upon dumping, there was a dull crack, and water
and (luckily already cooled) very finely granulated metal was
propelled explosively out of the pot. Most of the metal stayed in the
pot, but some was recovered from the wall, table, exhaust and my hair.
The granules varied in size from jagged bits 2-3 mm across to
something almost like flour.
Don't understand quite why it happened, but it is a good argument for
remembering wearing the safety goggles, even when doing something you
have done a hundred times and consider to be harmless.
Peter
(Now wearing a pair of fresh shorts!)
This happened som time ago, but I thought I would share it with you:
There is a discussion on this in Orchid archives. If link does not
work search for "Exploding water casting"
http://www.ganoksin.com/orchid/archive/200404/msg00558.htm.
I personally don't believe in the "spontaneously trapped water hypothesis" (I do believe in the explosion after trying one!) Surface tension and
difference in density should eliminate that, unless the water and the
metal are forced into contact.
When molten silver is poured into water, surface tension attempts to
draw it together in a ball (or burger, thanks to gravity) and the
water will not contact it directly, thanks to the Leidenfrost effect.
A thin film of steam is formed at the surface preventing the water
from touching the metal, greatly lowering the thermal transfer.
Se http://www.fysikbasen.dk/index.php?page=Vis&id=12 for some nice
pictures, and
http://www.wiley.com/college/phy/halliday320005/pdf/leidenfrost_essay.pdf
for a thorough explanation. (dropped it in the files folder too).
If a red hot silver coin is dropped in shallow water, it sometimes
just sits at the bottom making a buzzing sound for a couple of
seconds, then suddenly cooling with a sharp, almost clicking sound.
(or a drop of water racing around on a hot stove, most people know
that effect)
This is just a theory, but if the molten metal should start to
oscillate, breaking the film of steam, or be cooled locally,
collapsing the film in that way, the evolution of steam could push on
the metal, propelling it through the steam causing more explosive
steam formation, and the reaction would run wild. A local fast
formation of steam would certainly raise the pressure sufficiently to
powderize the metal and accelerate some water out of the container
(the energy is present, its just a question of releasing it fast enough)
http://www.ganoksin.com/orchid/archive/200404/msg00558.htm
"
"80 or so grams of it momentarily pool on the canvas, spread out as
three separate globules, then retract into the centre again
Certainly sounds like oscillation to me.
I superheated the metal, end dumped it as hard as possible, which
could have forced it to spread and contact the water (that was in fact
what I intended, I certainly got what I wished for!).
If we can get a clearer picture, we might reach a testable hypothesis
(I like blowing things up, but prefer that it happens when I intend it
to! And cheaper things may make a better boom if thats the purpose!)
Peter