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Crystal Size in Metal


#1

All,

I have a bunch of questions concerning the way metals crystalize
when they transform from a liquid to a solid. Which cooling method
forms the smallest crystal size - fast or slow? Please describe in
plain terms what you mean by a slow rate versus a fast rate.

I also have a bunch of assumtions based upon my minimal knowledge of
metals that may be incorrect.

#1 - I assume that the smallest crystal size of the metal will yield
a stronger casting that will be easier to work.

#2 - I assume that to best control the rate of cooling you want the
temperature of the flask to be near the temperature of the molten
metal when you cast.

#3 - I assume that the alloy within the metal will not greatly
effect the size of the crystals formed in the casting.

#4 - I assume that when we alloy a metal the alloy always mixes 100%
with the crystal structure of the metal we are alloying. Which means
the crystal size will be uniform through out the casting.

I could be way wrong on these assumtions - so straighten me out.

Gerry Galarneau


#2

Hi Gerry; It has been my limited experience that yes, a smaller
crysal lattice in the metal does result in a stronger and more
ductile metal, as for speed of cooling if I’m casting stones in place
I imeadiatley put the flask back into the oven and ramp down about
200 degrees per hour, (probably a better way but thats the way I was
taught and I haven’t fogged a stone of late.,) My idea of allowing a
flask to cool slowly is to cast my silver flasks at 1150 to 1250
degrees for thin pieces this is quite a bit below the liquidus stage
of the metal (DeOx Silver) a good 600 degrees or more. Then let the
flask cool until there is no visible color on the button, this can
take from 10 min. to as much as 25 if it were a flask with a large
button, some one here on the board recomended placing a just cast
flask on some discarded wax and then covering the button with wax as
well, and I tried it ONCE and only onece, after I got the flame out
and the smoke and residue out of the shop I promised not to try that
again, it’s amaxing how low the flash-point of injection wax is when
placed on an 1600 degree surface.

Now on assumption number 2, I tend to go in a different direction,
My Theroy is that the longer the metal is at it’s liquidus state
which is sometimes near the point of becoming gaseous as with some
of the anti firescale alloys, ( remeber solid to liquid to gas) you
actually want the initial 600 degree for thin parts to 850- to 1000
degree difference to start cooling quickly to stop gaseius
entrapments which will turn into gas pockets and form porosity (or
even worse denditic porosity whilst it can evoke some lovely
patterns reminicent of fine lace agate) Many clients find this
unacceptable. so let your temperature drop a bit at the end of your
burn out cycle. but please remember that different alloys do react
differently,

Number 3
Not so Some how given the difference of the atomic weights and
crystal latice structures of some of the new alloy materials like
Indium, Boron, Silicon and several other modern alloy materials I
don’t think they could help but change the not just the size of the
molecular grain but the lattice structure as well, We have a couple
chemical and Scientific heavyweights on the board, Mr Ballard from
Precious Metals West and our very own Man from Down Under John
Burgess, These gentelmen can actually shed some true scientific
knowlege on the subject. My knowlege just comes from alot of years of
casting and making changes, Though I’m not sure Mr Burgges is up to
it yet. (sure am glad to hear he’s recovering so well)!

Number 4
That sounds so nice and I wish it were always true but alass this is
a less than perfect world, and unless you are operating extreemly
sophisticated closed environment induction melting system it will
happen most of the time, and some times you may get lucky and not
scorch the metaL I’ve used a ker Electro melt with mixed results,
even an old Hoover melting system, and I’ve run Induction systems,
as well as Oxy Accet., Induction has the best chance of yielding a
homogenous mixture, but with care and practice you can get pretty
close with a big gas torch…

( DON’T YOU WISH WE COULD ALL PLAY WITH SILLY PUTTY AND ROCKS FROM
THE SIDEWALKS?)

Kenneth Ferrell
www.shadras.com


#3

This question opens a large can of very active worms. I had a slew
of classes in this very topic about 20 years ago, and can remember
very little of it right now (), but in general, fast cooling
yields small crystals (frequently you need to actively quench).
BUT - depending on the alloy you’re dealing with, cooling rate,
pauses in cooling, and re-heating may give you a metal whose texture
consists not only of different size crystals, but also of crystals
of different compositions. That is - you don’t always end up with
grains of all the same thing that reflect the gross composition of
your alloy. When held at a particular temp, some components may
exsolve and create grains of composition A within a matrix of
composition B, neither of which reflects the exact proportions you
started out with.

This behavior is the subject of lifetimes of specialization of
metallurgists, and thousands of pages of phase diagrams. Make it
easy - find out and write down take your precise alloy composition
and go to a metallurgist, who is paid to know all this stuff
(there’s bound to be one at any good engineering college near you).
She will think a moment, take a thick book off a shelf, and find
your alloy system’s phase diagram, from which she will be able to
tell you chapter and verse what to do to get what you want.

Tas
http://www.earthlywealth.com/


#4
    I assume that the smallest crystal size of the metal will
yield a stronger casting that will be easier to work. 

The smaller the crystal the more ductile and stronger the matrix

I assume that to best control the rate of cooling you want the
temperature of the flask to be near the temperature of the molten
metal when you cast.

No, you want the flask to be as cold as possible and still be able
to fill the flask. The cooler the flask the faster the
solidification so there is less time for metal - mold chemical
reactions to take place therefore less porosity and smaller crystal
size.

    I assume that the alloy within the metal will not greatly
effect the size of the crystals formed in the casting.

It has a great deal to do with the crystal size. Different alloy
components = different crystal size. In fact there are certain alloy
components that are added to control grain size they are called
grain refiners. For example iridium is sometimes used as a grain
refiner to keep the crystal size down in red gold alloys.

    I assume that when we alloy a metal the alloy always mixes
100% with the crystal structure of the metal we are alloying. Which
means the crystal size will be uniform through out the casting. 

No , some alloy systems form a complete solid solution , they are
completely intermixed with each other in a homogenous fashion.
However many alloys have two or more phases present at room
temperature. Sterling silver as cast is a good example it will have
a matrix of alpha phase (silver rich crystals that are 91.2% silver
or greater) a small number of beta phase (92% or more copper) and
the spaces in between them filled with the eutectic phase which is
28.1%copper and 71,9% silver. So three different concentrations in
one alloy. The phases present in an alloy are very temperature
dependant. A good experiment to show this is to dissolve sugar in
water untill you cant get any more to dissolve, then heat it up you
will be able to get more sugar into solution at higher temperatures.
As it cools the sugar will fall out of solution. This is exactly how
different metals act as they are alloyed and heated and cooled. A
Binary Phase Diagram will show the temperature and phase data for
two metals across a complete range of composition.

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau


#5

And kids wonder why they need to take physics in school! Your
explanation was wonderful Jim - thanks! I wonder how many lay
people have any idea of the knowledge needed to really do a good job
with casting, soldering, etc.?

Beth in SC


#6

Beth,

And kids wonder why they need to take physics in school!  Your
explanation was wonderful Jim - thanks!  I wonder how many lay
people have any idea of the knowledge needed to really do a good
job with casting, soldering, etc.? 

That is exactly why it can be so valuable to attend conferences like
the Santa Fe Symposium. I attended for several years, and managed to
acquire a rough-and-ready grasp of metallurgy and the dynamics of
casting, even though most of the time I only understood approximately
25 percent of what was said! Plus I met lots of really cool people I
can call for an explanation when I don’t understand something –
which is often!

Yet I have heard many jewelers scoff at the Santa Fe Symposium,
saying it’s “too technical.” Jim’s excellent post demonstrates why
sometimes it’s good to wander into the realms of the academic and
technical!

Thanks Jim!
Suzanne
Suzanne Wade
Writer/Editor
Phone: (508) 339-7366
Fax: (928) 563-8255
@Suzanne_Wade1
http://www.rswade.net


#7

All,

Thanks for the explaination about the crystallization of metal. I
now have abetter understanding. Still there are lingering questions
about the application of this knowledge.

Question 1 - Does each alloy have a specific melting temperature
versus flask temperature to asuure the “best” casting?

#2 - Is there a great advantage in casting strength using casting
shot alloyed at a large facility over using .999 gold coins and
alloying it yourself?

Gerry Galarneau
gggemswcr@cox.net


#8

Gerry,

Casting is a combination of Science and Art. The problem is that you
can drive yourself crazy trying to understand every little
scientific detail.

Two people following exactly the same procedures with the same
materials can have different results.

Any combination of things can change the way a casting comes out.
Whether you are using an open flame or a controlled atmosphere
furnace to melt your metal, Altitude, alloy, temperature gauge on
your oven, type of wax , length of time the flask cools, burnout
cycle, type of Investment, etc., etc, etc. all have an effect on the
outcome

The point I make is that you start experimenting with your equipment
using different burnout cycles, alloys, Investment, wax etc until
you find the combinations that work best for you and your equipment.

It is good to have a starting point by asking questions, reading
books and taking classes but nothing takes the place of
experimenting.

Good Luck
Greg DeMark
email: greg@demarkjewelry.com
Website: www.demarkjewelry.com
Custom Jewelry - Handmade Jewelry - Antique Jewelry


#9
   Question 1 - Does each alloy have a specific melting
temperature versus flask temperature to asuure the "best" casting? 
   #2 - Is there a great advantage in casting strength using
casting shot alloyed at a large facility over using .999 gold coins
and alloying it yourself? 
  1. Yes each alloy has its own melting temperature, and flask
    temperatures should correspond with the weight of the piece being
    cast. Typically the larger and heavier the pieces, the lower the
    flask temperature and the opposite.

  2. The only advantage you have by buying alloyed grain is that it is
    most likely to be done under a controlled environment with more
    accurate results. However you do pay a premium for alloyed shot, if
    you can shot out metal yourself with good results in a controlled
    environment I would suggest such.You must be careful not to over or
    under carat your metals, and make sure you have a good mixture or
    you may end up with hard or soft spots.

Right now I would suggest starting off buying from a well known
vendor and once you become more comfortable and learn more about
metals and the alloying process in general, begin working on your own
alloying procedures. Accuracy, control, and consistency are the
keys.


#10

I agree with Greg. Casting is a string of variables, any of which–
singularly or in combination with others-- can become problems.
Eliminating variables when ever possible is always a good idea. This
may include weighing investment, calibrating kilns, mixing with
distilled water, etc.

That being said, I firmly believe that a functional understanding of
what’s going on as each step of the casting process occurs is key to
trouble shooting. Recognizing, for instance, the shape and
appearance of casting flaws such as tears or pits and then
understanding how and why they form can help you track down just when
in the process they may have been generated (investing, burnout,
casting or cooling). When you know where to look first, trouble
shooting is that mush easier.

Andy
coopermanjewelry.com


#11
    Does each alloy have a specific melting temperature versus
flask temperature to asuure the "best" casting? 

Yes, the super heat (temperature above the liquidus the molten metal
is raised to) and flask temperature needed will vary depending on
the cross section of the items being cast and the fluidity of the
alloy at a given temperature. With that being said I have not seen
much data to help us determine what amount of super heat and how hot
to run the flasks. Generating this kind of data is tedious and would
require numerous casts of each alloy for each cross section chosen
to gain a statistical database large enough to really provide good
info. It is the kind of thing one assigns to materials sciences grad
students but the jewelry industry has not supported that kind of
research so we have some very general numbers from the alloy
manufacturers, investment suppliers and how to books that probably
were copied from someone else’s published data who probably borrowed
it from someone else. There are a few studies that you can find by
looking at publications from the Santa Fe Symposium and from Gold
Technology (which unfortunately is no longer being published) Along
with some papers and articles published in the most recent Stuller
metals catalog and Hoover & Strong’s catalog (many of which were
first presented at the Santa Fe Symposium). From these you can get
an idea of temperatures but they are for a limited number of alloys.
It is however a starting point.

       Is there a great advantage in casting strength using
casting shot alloyed at a large facility over using .999 gold coins
and alloying it yourself? 

It depends, there are alloy suppliers that are making up their
casting grain pretty much like most small shops would and have the
same defects in it that a small shop melting with a torch or even a
small furnace will. Such as poor mixing of the alloy components,
high oxygen content, water in the shot from poor graining practice
high levels of nonmetallic inclusions . Unless you are invested in
sophisticated equipment and carefully monitor your alloying and
graining it is hard not to make this kind of shot. Will that cause
you problems ? Yes, will you notice them? It depends on how good
your casting practices are if you have other problems with your
process then poorly produced alloy may not be all that noticeable as
a area of problem, It will depend on how big a problem you have with
your casting process. If you have a good casting process in place
then yes you will definitely notice a difference in alloy quality
There are alloy suppliers that are running very tightly monitored
manufacturing processes and produce very high quality casting shot
that will not cause you any problems when you use it.

Is it worth the expense? depends on how much you value your time.
Say you have cleaned a casting, soldered , pre-finished and set it
and now are in the final finishing and you polish down into a
nonmetallic inclusion that leaves a nice big fat comet trail on a
nice wide flat surface because the alloy supplier was not too
careful with the cleanliness of the materials going into the melt.
Or you are fighting lots of gas porosity in the casting even though
your melting process was very clean because the vendor did not
control the amount of oxygen and other dissolved gasses in their
product.

I am not saying that you will never have problems with your product
if you only use good quality alloy but you will have less of them.

We used to periodically get salesman coming by the casting shop I
worked at who would offer to provide us with less expensive shot
than we were buying from our supplier. The boss would often take
them up on a sample lot and we would try it out. Most of the time
the stuff was a problem in one way or another and we would never buy
another lot from them. I really loved the stuff with the water
trapped in it and you get the little steam explosions as it melts
and you just know you are going to have porosity problems :frowning:

Hope this helps,
Jim

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau


#12
Does each alloy have a specific melting temperature versus flask
temperature to asuure the "best" casting? 

Well, sort of. We know the lowest optimal flow temperature for most
alloys. But superheating of fifty or one hundred degrees F. can
sometimes help filling, sometimes at a cost in surface finish. Ditto
for flask temperatures. It’s a system with your sprue size as
another unmentioned factor in your question.

Is there a great advantage in casting strength using casting shot
alloyed at a large facility over using .999 gold coins and alloying
it yourself?

There is no strength advantage. You might save your metal one melt,
(controversial point) but you can certainly manage your grain
inventory better. Example-Monday you cast 10ky for a customer.
Tuesday you need 14kt, so you add the correct fine gold to come up to
proper karat, and hopefully, fresh 24kt and alloy for 50% fresh
overall.

Daniel Ballard
PMWest


#13
        Does each alloy have a specific melting temperature versus
flask temperature to asuure the "best" casting? 
   Yes, the super heat (temperature above the liquidus the molten
metal is raised to) and flask temperature needed will vary
depending on the cross section of the items being cast and the
fluidity of the alloy at a given temperature. 

There are a number of other factors as well. The degree/speed at
which a given metal radiates it’s heat away, which is in part a
factor just of it’s melting point, but also, I think, of the metal’s
density, plus the insulating ability of the casting investement
used, both also affect the speed at which a given molten metal will
solidify in the mold. And the fluidity of the metal being cast will
affect the speed at which it can fill a mold during casting. The
ultimate goal is to have the metal fully fill the mold cavity at the
lowest possible temperature (both mold and metal), in order to
acheive the lowest amount of porosity and casting flaws, as well as
the densest casting with the smallest and most uniform crystal
structure. But I really don’t think there are any one set of best
temps and settings. Too many variables. Things like the exact
details of which type of investment you use, and how you mix it
(changes it’s density/porosity/gas permiability, and thus the speed
at which air is removed in front of flowing metal, as well as the
insulating properties of the investment, which changes the rate at
which the metal will solidify and cool), the exact method you’re
using to melt and pour the metal, the exact temps and times of your
burnouts and the exact nature of the atmosphere in your burnout
furnace, and the number and nature of your waxes, as well as how
they’re sprued: all of these may affect the ‘ideal’ temperatures at
which to cast a given alloy. You can come up with general
recommendations, and these can be good starting points.

But the simple truth i’ve found is that ideals and recommended temps
and times and all that, are very much a “your mileage may vary” sort
of thing. Each caster has to play with those starting points and
recommendations to fine tune their own process. Keep good notes on
what you did, what you did differently, and how it affects the
results, and you’ll be able to zero in on your own best settings for
given types of castings. What seems to work best for you may not do
so for someone else, nor may their own seeingly ideal settings
always lead to success when you duplicate them.

Peter