How to Grow a Large, Single Crystal: Part 2 (Turning Your Seed Crystal Into a Large Crystal)


UNIVERSITY AT BUFFALO CHEMIST JASON BENEDICT:
This is the second video in a two-part
series on how to grow a large, single crystal. In the first video, we showed you
how to grow a seed crystal. The second video is going to show you how to turn
this seed crystal into a large, single

[Benedict in a chemistry lab.] crystal. And so what are we looking for
in a large single crystal? Well we want large, flat faces, sharp edges. And we’d
like the crystal to be as

[Benedict holds up a crystal the size
of a ping pong ball.] see-through as possible. And
we also want it to be a single crystal — just one crystal. And that’s what
we have here. To turn our seed crystal into a large, single crystal, we need to
place our seed crystal in an alum solution. As the water evaporates the
alum needs to come out of solution, and if our seed crystal is present, the alum
will stick on to that seed and cause the seed to grow. Now there are a lot of
different ways to put a seed crystal in

[Benedict in a lab.] a solution of alum. I like to use fishing
line. Here we have 0.3 millimeter fishing line. You can pick this up at any
sporting goods store or craft store. Now,

[Benedict holds spool of fishing line.] you may have seen in some of my other
videos that I tried to glue the fishing line to a crystal, and it didn’t really
work out so well. So my preferred method is tying, and the way that I do this
is I actually get my string, and I tie a slipknot. If you’re not sure what a
slipknot is, search the web and find out.

[Benedict holds a fishing line about
10 inches long, with a looped slipknot
at one end.] But, basically, it’s a nice little loop
that’s going to grab on to whatever you put in the middle. So we have our we have
our slipknot, and we have our seed crystal. We can just sort of fish our
slipknot onto the seed crystal. There we

[Benedict puts knotted fishing wire
around seed crystal.] are, come on you. And there
we have it. We have

[Benedict tightens looped fishing
wire around crystal by tugging on
the wire.] our crystal securely fastened to the
string. We have this little extra piece of string. We can go ahead and grab
a pair of scissors and just carefully snip that away. And now we have a crystal that’s ready to be submerged in

[Scissors snipping excess fishing wire
near slip knot loop. You should not snip
off the long end of wire.] a saturated solution of alum. So it’s
actually a good idea to prepare a few seed crystals, because we’ll be taking
these seed crystals and putting them

[Benedict holds several seed
crystals tied to fishing line.] into a solution of alum, and a lot can go
wrong. And so you may need a backup seed crystal, and I’ll get more
into the details a little bit later. You may remember from our last video
that we harvested seed crystals from our large, flat dish. We don’t want to put the
seed crystals back in here because, as you see,

[Benedict gesture toward large, flat dish
on lab counter.] there’s lots of other crystals around. We
want to heat this solution up to get all

[Overhead view of dish showing
small clear crystals at bottom.] of these crystals to dissolve. And so to
do that, in this case we’re going to want to transfer it to a container that is
suitable for heating. So we just carefully pour this in, and this is again
where a squirt bottle can come in handy

[Benedict pours solution from
flat dish into a glass beaker.] to help rinse some of these little
crystals in there. Now I’m not going to

[Benedict uses squirt bottle to spray
water on bottom of dish to loosen
small crystals, which are also
poured into the beaker.] spend 20 minutes getting all those
little crystals out, but I recommend that you do. Now, we’ve added a little bit more
water back in here through the process of cleaning our dish. But how much water
do we want? The short answer is, we definitely want less water
than that. This blue line indicated the

[Benedict points to small blue line
on side of beaker.] amount of water that we used to initially
grow the seed crystal. And the idea is, if we put that much water back in here, it’s
just gonna redissolve our seed crystal. So we want a little bit less water than
that, but exactly how much? That’s

[Benedict in the lab.] something you’ll have to experiment. Too
much water, and the seed crystal’s going to dissolve. If there was too little
water, the alum is going to want to crystallize out incredibly rapidly and
could potentially destroy the seed crystal. So you want just a little bit
less water than you started with for your seed. So we’ll go ahead and take our
container put it on the hot plate and let this start heating. If you have a lot
of solid material down there, you might consider stirring this, but in our case,
there’s not much left, so we’ll be fine without a stir bar.

TEXT ON SCREEN:
Heat the solution to about 45 to 55 degrees Celsius.

When all of the alum has dissolved, let the solution cool. We want to get an ideal crystal shape.
And the reason we suspend the crystal in the solution is because if a crystal
grows against the bottom of a dish or against a surface,
you’re not allowing the crystal to grow equally in all directions, and that’s
what can give rise to some of the irregularities here. These crystals
likely grew on the bottom of the dish.

[Five odd-shaped crystals, including
flat ones.] Again, when the crystal grows against a
surface, the crystal can’t really grow in that direction, and so you wind up with
generally very odd-shaped crystals. When the crystal is suspended in the middle
of the solution, all of the directions of the crystal have the ability to grow,
and very often that will lead to crystals with the ideal crystal shape. So
one of the things that can go wrong is

[Close-up of large, single crystal.] adding your seed crystal to a warm alum
solution. Now, if the solution of alum is

[Benedict in the lab.] warm, it’s almost certainly going to
dissolve your seed crystal. And that’s

[Footage of small crystal shrinking
in a warm solution.] what you’ll see here. So here is our
cooled alum solution, to which we want to add our seed crystal. Now, how do we
suspend the seed crystal in the alum solution?

[Benedict with glass beaker filled
with clear solution.] I like to use some sort of an irregularly
shaped long thin object. If you use

[Benedict holds a plastic knife.] something round like a pen or a glass
rod, it can roll around. Using something flat helps hold this in place. And so
what we’ll do is we just attach our seed crystal to this. Whatever object you want
to use, take a little piece of tape and tape it in place. Cut off the excess
string, and

[Benedict attaches fishing line to
plastic knife. Seed crystal hangs
from bottom of the line.] here we go. We’re ready to submerge
our crystal. Now here we can see it’s

[Benedict places seed crystal in
beaker holding solution. Plastic
knife goes across top of beaker.] hitting the bottom. We don’t want the
crystal on the bottom of the dish. We

[Seed crystal hanging off the fishing line
sits on the bottom of the beaker.] want to raise it up a little bit, and so
that’s a nice thing about this cutesy little trick. We can just rotate this
knife a couple turns. It lifts the crystal up, and now the crystal is
suspended midway in the solution. And

[Seed crystal dangles in beaker
holding alum solution.] this is exactly what we’re looking for.
There’s no other crystals around. So now, any alum that wants to
crystallize out of solution is going to

[Benedict in the lab.] be preferentially attracted to the seed,
and the seed is going to grow. Crystals like to grow in an area where they’re
not going to be disturbed. Ideally, this will be a dark cabinet. It’ll be
dark, there’s no light. It’s going to be free of air currents. And one thing you might
consider doing is actually setting this on a nice piece of styrofoam to help not
only thermally insulate it, but also to

[Benedict gestures to beaker holding
seed crystal and alum solution.] potentially free it from vibrations that
could cause unwanted crystallization. So we’ll go ahead and put this in a dark
cabinet. So here we have an example of a

[Benedict lifts beaker up.] crystal that I’ve been growing for about
a week or so. It started out exactly like this — a single crystal and a beaker
without any other crystallites — but

[Two crystals growing in beakers. One
is in a clear solution. The other is in
a solution with small crystals at
the bottom.] here, a week later we can see that not
only has our seed crystal grown, but now we have a bunch of other crystallites
all around the beaker. The trouble is,

[Tiny crystals at the bottom
of the beaker.] these extra little crystallites are
alum crystals that are growing, and they’re going to take away from the
maximum size that your seed crystal can

[Benedict in the lab.] achieve. So how do we get rid of these
parasitic little crystals? What we’re going to want to do is pull our crystal
out of solution, reheat the solution, and let it cool, and then put our crystal
back in. The first step is to pull the crystal out of solution, and like I
mentioned last time, you’re going to want to squirt this off to prevent any
possible little crusties from forming on

[Benedict lifts the crystal out of the beaker
and sprays it with water from a squirt bottle.] the surface of the crystal. And we’ll
just go ahead and set this to the side.

[Benedict places crystal on
paper towel.] So we have our solution. We’re going to
put it on a hot plate.

[Benedict holds glass beaker.] And this is where we’re gonna want to
pay attention to our blue line again. So once again, the amount of water is
critical. This blue line represents the amount of water that was present when I
first introduced the seed crystal. We’re

[Benedict points to small blue line
on the side of the beaker.] going to want to add back in some water
to this solution, but not quite to the blue line. The reason you want to add a
little bit of water back into this solution is we have a lot of extra alum
at the bottom. If we didn’t add any more water, when this solution cooled back
down, all of this alum would want

[Benedict in the lab.] to rapidly come out of solution, and it
would it be like a snow globe. There would be alum powder everywhere, and it
could really jeopardize the appearance of our seed crystal. Now, we’re just going
to heat this up until all of these crystals dissolve. We’re going to let this
solution cool back down, and once the solution is returned to room temperature,
will reintroduce our seed crystal. Okay. We have our cooled solution, and
again, the liquid is a little bit lower than our original blue line. And so we can go
ahead and add back in our nice alum

[Benedict points to water level, which is
lower than the blue line on the side of
the beaker.] crystal. Now at this point I’m going to
take my trusty marker and add a new blue line. The second blue line is a
convenient reference point, because as we

[Benedict draws a small blue line
to mark the water level in the beaker.] let this crystal sit, water is going to
evaporate, and at some point we’re going to get more of those little parasitic
crystals that we need to get rid of: We’re going to have to remove the
crystal, reheat the solution, add back in a bit more water. And again, we’ll know
how much water to add because of our second little blue line. So now we’re
going to take our crystal and put it back

[Crystal hanging in beaker filled
with solution.] in our dark cabinet and let it grow
until we start to see more of those parasitic crystals, and we’ll have to go
through another round of removing the crystal and heating. This is something
that you may have to do three,

[Benedict in the lab.] four or five times. You basically are
going to have to do this every time you see a lot of little extra
crystals growing in your beaker. But every time you do
this, there’s a chance that something can get messed up, and you’ll destroy your
single crystal. So you want to do it as few times as possible. But inevitably,
you’re going to have to do this. But for now, into the dark cabinet it goes. Once
you have your large crystal going and

[Benedict lifts beaker holding
single crystal.] you’ve been through maybe one or two
cycles of removing and heating to get rid of parasitic crystals,
it’s probably a good idea to take any of the leftover seed crystals that you have
and toss them in there when you do

[Benedict holds up a seed crystal.] another heating cycle, so that you make
sure you have all 100 grams of alum at play in the beaker. So that concludes our
two-part series on how to grow a large,

[Large crystal dangling in beaker.] single crystal. In a few weeks, with any
luck, this crystal will turn into a

[Benedict in the lab.] crystal worthy of winning the U.S. Crystal
Growing Competition. Thanks for watching.
Good luck with the crystal growth! [Lighthearted music plays.]

[University at Buffalo logo on
blue background.]

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