>> Today I will give a
demonstration on the mixing
of zinc phosphate cement.
And the reason that I'm going
to do this with video is
that it's much more
clearly demonstrated
with a video camera live,
than it would be from slides.
And as you might know from
your reading, the proper mixing
of zinc phosphate cement
is really critical in terms
of the final properties
of the material
and that you gain the
proper working time.
The armamentarium that's
needed are the bottles
of zinc phosphate cement,
this is the phosphoric
acid liquid right here,
and the powder is principly zinc
oxide and some magnesium oxide,
minor amounts in
the powder itself.
In addition, you need
your cement spatula,
stainless steel cement spatula
that us see here, and finally,
a cement mixing slab that
has been cleaned and dried,
and preferably one that has
been refrigerated or put
into the ice bath
for a period of time.
One has to be careful however,
I placed one in the freezer,
just to demonstrate, if it's
too cold, then I don't know
if you can see that clearly,
if it's too cold you can see
that moisture forms
on the surface
because of the dew point.
And you can then see by wiping
here, that moisture is removed.
One has to wait until
the glass slab arrives
at a higher temperature so
that moisture does not form.
And then the mixing can begin.
You can look up to see what the
effect is of surface moisture
on the setting of the
zinc phosphate cement,
but it will have an
effect on the setting.
Initially, no matter what one
mixes, where there's a powder,
you should fluff the
powder to some extent
so that it's not too dense.
The next step is to
place two cap fulls,
and it's not really an entire
capful, it's to the top
of the dome where it becomes
flat internally and I'm sure
on the white here, it's a
little hard with the contrast.
But it's just to the top of the
cap that we'll fill two levels
of powder, as I'm doing now.
I'm squeezing the powder
and in the process,
and it's a little
hard to do it exactly
because it doesn't lie flat,
but you can get awfully close,
and there's one capful.
And then a second capful.
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Anytime you use either
powders or liquids,
remember to replace the cap
after you've dispensed
the material.
The next step then is
to create a flat surface
so that you can divide it up
into the proper increments.
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And this can then be divided
into half, then into quarters,
it's not quite a quarter there.
And this segment can be divided
into an 1/8, and then 2/16.
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Okay. Next, I'll
dispense the liquid,
and in dispensing the liquid,
you hold the bottle vertically
and gently squeeze, and you'll
see the droplets forming
and you count one, two, three,
four, five, six, seven, eight,
nine, ten, eleven, twelve.
Twelve, and remember once
again, to replace the cap
when you're using the liquids
because if water is lost here,
it'll alter the setting
characteristics of the material,
of the phosphoric acid and
the zinc phosphate mix.
Okay, our next step then is to
begin mixing, and you'll want
to review the times of mixing
and one handy thing to start
with is using a stopwatch to
begin with, because you'll note
that there are 15 second
intervals of mixing
and then it goes to 20
seconds and finally 35 seconds.
So, I'll use for my
purposes here right now,
the stopwatch just
to keep track.
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The first part is you take
in one of the 16th elements
and you dispense if
over a wide area,
or mix it over a very wide
area, to dissipate the heat
of the reaction and then the
objective is to thoroughly mix
in the zinc phosphate, the
zinc oxide powder and then
after 15 seconds, you
take in one more 16th,
and you'll notice I'm using
kind of a stropping motion
over a wide area thoroughly
incorporating the powder.
And now at the end
of 30 seconds,
I incorporate the
1/8th segment--
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-- thoroughly, for 15 seconds.
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And then 1/4 segment
for 20 seconds--
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And then another 1/4.
You'll notice I take a little
care with the initial entry
of the powder and the liquid.
It wouldn't take much
with either some error
to blow a portion of it
away from the glass slab.
And this is for 20
seconds again.
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And then the final
increment is incorporated.
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Sometimes the ratios are not
exactly perfect and you may need
to add a little powder
on the slab,
that if the viscosity does not
turn out to be just quite right.
Okay, now is the complete 2
minutes and as proof of that,
I'll have Bill zoom in
on the watch right there
and what you do at
this stage then is
that you gather the
cement together,
and then you test its
viscosity, and it should string
about 2 centimeters, it comes
up just a slight bit short
and what one can do then is
add a tiny bit of powder,
over in the corner
here, incorporate that.
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You can take a small
amount of powder--
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And that should render
the proper viscosity
for a 1-2 centimeter
mix, about like so.
That is pretty much ideal, and
I'll hold this to the side again
so that you can see
as I lift it.
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Okay, that pretty much completes
just the mixing technique
for zinc phosphate cement.
One thing that's important
too is the proper cleanup
and you can note the syllabus
instructions for that.
One thing you do want to do
after the crown has been loaded
and you proceed with
cementation,
then place the cement slab along
with the mixing instruments
in water, and typically we'll
clean up nicely in water.
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