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Pitting in cast metal


#1

My jewelry pieces coming back for the caster all have tiny pinholes
(pitting) in them. I have asked the caster why this would occur and
was told that the metal has sediment in it. I was told I need to get
a 'hammering tool" to fix this. I’ve looked high and low for such a
tool or technique with no luck. Is this the case with all cast metal?
I have cast in both brass and silver and my waxes are smooth so I am
not sure what to do other than just deal with it. Any advice on how
to fix this? or should I try another caster?


#2

HI Donatella,

Unless you supplied the metal, there’s no reason why "sediment"
should be your problem. (So there was crap in the metal they
supplied you. Just why, exactly did they think this would be an OK
explanation?) And that’s not really the problem. Sounds like they
massively overheated the metal. Either that, or bad spruing.

Lots of little pinholes all over sounds more like the metal was too
hot. If it was just in certain spots, that would be more-likely bad
spruing. Did you do the spruing or did they?

Either way, find a new caster. These guys are shining you on.

I’ve cast all sorts of random crap supplied by students, some of it
even with little bits of investment still on. Never had a problem,
even when I knew there was ‘sediment’ on the stuff I was melting
down. The trick is to be careful, and take the time to dig around in
the melt with a carbon rod to sweep all the crap out once it’s all
molten. Not what I’d do for high-value pieces, but in 12 years of
teaching casting, I’ve never had a bad cast due to inclusions.
Amazing what you can get away with.

The ‘hammering tool’ they’re talking about is probably a rotary
hammer burr for a flex-shaft too. Handy little bugger, but you
shouldn’t need it very often.

You can get them from the usual suppliers. I’ll include a link to
Rio so that you can see the picture. Rio calls them Margin Rollers.
Just run it slow, and use lubricant like liquid burr life, or
sewing-machine oil (if you have to.) Burr life is MUCH better than
oil.

Rotary hammer burr here:
http://www.ganoksin.com/gnkurl/ep81vm

Regards,
Brian


#3

I would find a new caster


#4

Even the best caster occasionally have pits. Have the caster supply
the metal. If the castings are continuously porous, find another
caster.


#5

Try another caster.


#6

What “sediment” ? That sounds a bit like a lame excuse to me.

Who supplied the metal, you or the caster or a third party?

Metal should be suitable for the intended purpose or rejected if
not.

That’s all I can think of unless the caster simply did a bad job. I
don’t know enough about casting to suggest more.

But if the casting is no good then you shouldn’t pay for it and find
another caster


#7

Donatello-Is the caster using your metal or their metal? Are they
using a torch to melt or a furnace? Usually when there are pits in
the cast pieces it’s from over heating the metal. Never heard of
"sediment" in metal.

Perhaps contamination is what they meant.

The hammering tool they referred to is a small rotary burnisher that
is used with a flex shaft.

We’re it me, I’d be very unhappy. If you have rubber molds I’d
reshoot the waxes and give them a second chance to get it right, but
only if they don’t charge for their labor. If they balk just walk
away and find a new caster.

Have fun and make lots of jewelry.

Jo Haemer
timothywgreen.com


#8

Hello Donatella

First the ‘hammering tool’ - it is as simple as using an ‘eye
bolt/screw’ inyour flex shaft. The open ‘eye’ allows you to see what
you are doing as the rotating metal beats on the piece.

However, I think you need another caster. Although you don’t
indicate what the metal is that ‘has sediment in it’, something is
wrong. Unless you provided the casting metal, the caster should be
using first quality metal without ‘sediment’!

Judy in Kansas, who is hoping this misty day turns to rain. The wheat
really, really needs moisture.


#9

Porosity can have several different causes, sometimes it’s a
combination of causes, but I’ve never heard of “sediment” as a cause
of porosity. My guess is that the waxes are not being gated right
and/or the metal is being cast at too high temperature. Generally
speaking, a caster that essentially tells you that you’ll just have
to live with it is one that probably needs to be replaced. You
shouldn’t have to live with it.

As to the “hammering tool”, you can use a hammer handpiece on your
flex shaft and use a regular, plain bit (tip, anvil, mandrel,
whatever you might like to call it) on which you have rounded and
polished the tip. Using a fairly high speed, burnish the area first
from one direction (say parallel to the shank) about 45 degrees from
the surface, and then at an angle of about 90 degrees to the first
(across the shank), still around 45 degrees from the surface. Finish
with sandpaper of progressively finer grit and then lightly polish.

The idea is to push or burnish the surface metal into the tiny holes
and then smooth and polish the roughened surface. If you sand or
polish too much you can go below the burnished surface into the
porosity and have to do the whole thing over again. It always seems
to get worse the deeper you go too, so avoid that if possible.

This should be a rarely used solution to porosity. A good caster
will be able to tell you what’s going wrong and help you make sure it
won’t happen again, assuming it’s something you are doing, like
incorrect gating or asking them to use too much old metal (more than
50%). If it’s something happening on their end, they ought to own up
and take care of it. If they won’t or can’t, you should find someone
else to do your casting.

Dave Phelps


#10

After no-fills (incomplete castings) porosity is the biggest casting
problem (not counting casting the wrong metal).

Porosity can begin with investing, burnout, melting, mold cavity
fill or in the cooling.

Discovering the cause of porosity is a matter of forensic sleuthing.
It begins with the shape, size and location of the pits. Here’s a
few from my exper -jagged pits. Most likely some particulate
inclusion, on the surface of the metal being melted, in the mold
cavity or n the crucible

-jagged or unsound pits filled with white powder. Usually this is
investmentthat has spalled or broken away from someplace in the mold
cavity. Often these begin as a small speck that grows and grows as
you file (and uncover more of the investment inclusion). Most often
these pits are accompanied by a complimentary positive or raised
roughened area in the surface of the casting. This is where the
investment spalled.

-tearing. So called Hot Tears most often occur in thin areas between
thickerareas. This is the result of stresses that occur during the
cooling of the cast metal.

-round/spherical pits. Gas Porosity. Trapped gasses in the molten
metal massoften the result of contaminants or over heating the melt
or oxidation. If using old buttons to cast with, be sure to
completely clean off any pickle from the cleaning process. Another
cause of this is the overheating of the investment during burnout.
Investment breakdown.

-Shrink Spot porosity: looks like "spongy areas of metal. Often
small spraysof tiny non-round pits that often show up during final
polish (of course). This is a result of cooling metal and less than
ideal spruing. Thicker areas which are the last to cool and fully
solidify are literally torn apart by theareas that cool first and
shrink, pulling metal from these liquid/semisolidthick areas. It’s
how the button is designed to work. It should be the thickest/most
massive area of the casting and will be the last thing to cool
andsolidify, providing a place to draw metal from as the casting
cools. This is “progressive solidification” which ideally confines
pits to the button.

-large or smaller pits on the surface surrounded by a bright ring or
halo. Usually rounded hemispheric depressions. The result of flux
which has gotten into the metal flow and occupied space on the
surface, hence the depression. The halo is from the flux preventing
oxides. Ideally, the metal slides out from under or above the flux
in the crucible: being heavier it centrifuges outahead of the
lighter flux.

Hope that helps.
Andy
Please excuse any typos-- curse my clumsy digits…


#11

Hello, If you had tiny air bubbles in your wax that could cause you
to have porosity in your casting. Otherwise it was the caster who
messed up. I would ask them to redo or find a new caster. Dan


#12
Hello, If you had tiny air bubbles in your wax that could cause
you to have porosity in your casting. 

I would be interested to hear how you think that works. Air bubbles
IN the wax are invisible to the process, leaving no mark or effect on
the mold. Air bubbles that reach the surface of the wax amount to
small holes in the wax, and these will reproduce in the casting. But
it’s not common for air bubbles to break the surface of the wax.
Usually they’re imbedded under a thin layer of wax. Unless the
vaccuum during investment manages to burst the bubbles, which is
uncommon, then again, the bubbles will have no effect. Now, if
instead of air bubbles in the wax, you have dirt or impurities mixed
into the wax, then perhaps that can leave some residue in the mold
cavity to mix with entering metal. But again, this isn’t a major
issue, since most of what would mix with the wax will also burn out.
The most common effect of air bubbles in casting is when an
insufficient vaccuum in investing leaves air bubbles in the
investment that adhere to the wax. These will reproduce during
casting, showing up as nodules on the casting. Most of the time, that
extra metal can be removed without too much trouble unless trapped in
inaccessable areas of the casting, or if they damage details of the
casting somehow…