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Red spots in homemade gold alloy


#1

I have only made my own alloy twice, but both times I have had a
problem with concentrated areas of copper showing up in the gold.
The first time it was basically one big spot that seemed to move
around each time that I annealed. Once I had a finished piece, I put
it in the magnetic tumbler and oddly enough the steel pins seemed to
sort of peel up a thin layer of gold in one small spot to reveal a
pretty large area of copper underneath. Even after melting this gold
again a few times, the red spot was persistent. The second time I
made an alloy, I had a few smaller red spots, which basically went
away after annealing and milling for a while. It seemed that the
spots just ended up being distributed by the mill so that they were
eventually not visible. Also prior to milling, I took my ingot and
melted it a little, but not to the molten state (I was really doing
this to add a piece of gold that was left straggling behind in my
ingot pour), which seemed to reduce the red spots a good bit.
Everyone that I’ve asked about this has never seen it before and the
typical suggestion is to melt it down again (which doesn’t do the
trick). Anyone have any insight into why these red spots are forming
and how to avoid them?


#2

Hi Annie,

You don’t say what alloy you’re making/using. I have (by fine
jewellery standards) a fairly cheap emerald and diamond ring in 9K
gold which was a present. I noticed some slightly different coloured
patches on the gold as soon as I received the ring, but didn’t send
it back as I really loved it. However, when I went to put it on the
other day, I noticed that the patches had got darker, so I inspected
them with my loupe. I too, discovered that the gold around these
patches is sort of lifting up around the edges. The whole casting is
very poor quality with these patches and lots of pitting. My ring
sounds like a similar problem to the one you’re experiencing. I’m not
sure what the solution would be. Do you stir your melted metal well?
Do you add borax (I think that’s the right thing to add) to the melt
to remove impurities?

I hope you get some answers.

Helen
UK
http://www.hillsgems.co.uk
http://helensgems.ganoksin.com/blogs/


#3

your alloy is Not mixed well enough - a graphite/ carbon stirring a
rod will help as will a pinch of 3:1 charcoal powder and sal ammoniac
as a refining powder to yield a bright tough ingot…also dedicate
your crucible to the various metals ( and I even dedicate them to
karats) so there isn’t cross contamination if you don’t use
disposable crucibles every time (I sell little bonded graphite
crucibles that are so cheap you can toss them or crush and reuse for
loads of things once the flux glass is removed (that goes into the
refining scrap as there are sometimes small granules of metal that
stick to the crucible’s ceiling in the borax glaze if you use
high-top crucibles, or even the sides in some models. After a few
uses the flux glass from borax gets discoloured with oxides from the
alloy’s components so at least reglaze your fused clay crucibles
periodically if you want a modicum of assistance in keeping them
cleaner- the downside is overglazing to where the metal is swimming
in a sea of melted borax…You can also wash the “air” around the melt
with a covering of argon gas- MAKE magazine has an article on
building a " rigged " melting environment (actually more trouble than
it’s worth without building an electric melting furnace too, ; you
have to know how to deal with open flames and argon - or any gas for
that matter, so the process of flushing the chamber with argon then
quickly bringing the warmed crucible and warmed metal to pourable
temperature is a learned art/skill ). Keeping dedicated crucibles is
the easiest most cost effective method i know of.


#4

You probably need more heat in your melting and something to agitate
the molten metal well otherwise it will separate. When molten and
well stirred, pour into a cold skillet or mould to prevent dendritic
growth of each component of the alloy.

Nick Royall


#5

I was making 18k yellow, and yes I added a pinch of borax to the
mix. I stirred only by swirling the crucible around a bit. I didn’t
know that it was ok to actually stick something in it to stir with,
nor would I have know what kind of “stick” to use. Thank you, Rourke,
for the suggestion.

When molten and well stirred, pour into a cold skillet or mould to
prevent dendritic growth of each component of the alloy. 

I’ve always been told to heat the ingot mould before pouring so that
the molten metal doesn’t splash (or something like that) when it hits
a cold mould. Are you saying that you are doing the opposite? Could
you explain this further?

I sell little bonded graphite crucibles that are so cheap you can
toss them or crush and reuse for loads of things once the flux
glass is removed 

Where can I buy these? I did dedicate my crucible to 18k gold only
(but it does actually seem to be overglazed now, even though I
haven’t used it that much).

Thanks for the suggestions!


#6
I've always been told to heat the ingot mould before pouring so
that the molten metal doesn't splash (or something like that) when
it hits a cold mould. Are you saying that you are doing the
opposite? Could you explain this further? 

I did say to heat the mould - absolutely before the pour. thermal
shock is common in hand pouring metals. Just lubricate the mould with
oil or beeswax and then heat, If you use a crucible with a hole ion
the back end for pouring the flame will shoot out the end if
positioned correctly heating the mould as you heat the metals. When
the lube smokes a bit it is hot enough to pour (i’m talking about a
two part mould with a handscrew on a U shaped clamp).

I also said to use a stirring rod. But not the skillet part - that is
wholly unnecessary unless you are melting a large quantity and using
a pre fab deoxidizing alloy (like other vendors selll to make
karating your scrap or casting grain more easily prepared in both
colour and uniformity from batch to batch). I would definitely not,
nor have I ever heard of in over 35 years pouring metal into a cold
skillet…double melting is unecessary if you remove the impurities
either by thoroughly mixing the alloy or stirring wioth a rod to pick
up the visible “black spots” clinging to the molten rolling ball of
your gold.

Some use a hardened quartz rod - I hate them. the carbon or graphite
ones are far more effective, but you do need a pot holder or glove as
they heat up in the quick stirring required to mix and remove the
bits of junk swirling around in your crucible.

The small crucibles are available from a variety of sellers including
harbour freight- if you get them there inspect for cracks or defects
before glazing them, and they require tongs to hold them- most people
dont use the tongs correctly, you don’t use them by bringing the two
ends together, but bend them inside each other to make a secure
holder for the crucible style you like best- they are then pryed
apart to grab the crucible and work great with burno style, to any
high back crucible or the vessel like graphite ones I was talking
about (if you can’t find em contact me off list I sell them rather
inexpensively).

Overglazing is easy to do. To correct: heat the crucible to red hot
and pour off all or as much as is possible excess into a bucket or
container of water with a submerged brick in it to give a flat
landing pad so to speak - that makes some great blue iolite or
sapphire looking practise cabs to use for teaching/learning bezel
setting!). then reapply a fresh coating of borax and a pinch of boric
acid to reglaze (i mean a small pinch ; less than an eighth tsp.

Hope this clarifies it for you.
rer


#7
I didn't know that it was ok to actually stick something in it to
stir with, nor would I have know what kind of "stick" to use.

A graphite stirring rod is the thing to use apparently. I also need
to get one of those, as I have just been swirling the metal in the
crucible to mix it.

Helen
UK


#8
I was making 18k yellow, and yes I added a pinch of borax to the
mix. I stirred only by swirling the crucible around a bit. I
didn't know that it was ok to actually stick something in it to
stir with, nor would I have know what kind of "stick" to use. Thank
you, Rourke, for the suggestion. 

Use either a graphite or quartz rod to stir your alloy when melting.

I've always been told to heat the ingot mould before pouring so
that the molten metal doesn't splash (or something like that) when
it hits a cold mould. Are you saying that you are doing the
opposite? Could you explain this further? 

First yes you always want to pour into heated metal molds. The
reason for this is to insure there is no water present. Even the thin
film of water that forms from condensation can cause small steam
explosions that can spit the metal out at you worst case or just
cause bubbles in the ingot.

Second the assertion that by pouring into a cool iron mold will
prevent dendritic crystal formation is just wrong. Metals solidify in
dendritic crystal structure unless you are able to super cool them on
the order of millions of degrees per second, thereby creating an
amorphous metal (no crystal structure). These cooling rates can be
reduced by some special alloys and processing techniques but still
way beyond a goldsmiths studio practice. This is the realm of R&D
laboratory science and is not used for anything but the most exotic
materials yet. If you are curious about these materials My friend
Boonrat Lohwongwat is one of the main researchers in them and you can
see some more on them by going to
http://tinyurl.com/mzwdtt [PDF file] or by Googling Boonrat
Lohwongwat

The poster may be referring to coring where the alloy segregates as
it cools making for different compositions as you traverse from the
inner dendrites that form early in the solidification process to the
outer dendrite surfaces that are the last to solidify. But even then
in a metal ingot mold there is no significant coring in the size
ingots we work with. Coring is often seen in industrial size ingots
it also shows up in investment or sand castings on the jewelry
scale. It takes time for this kind of segregation to occur and metal
molds cool too fast for any significant coring to occur in this small
scale.

Jim
James Binnion
James Binnion Metal Arts


#9
The small crucibles are available from a variety of sellers
including harbour freight- if you get them there inspect for
cracks or defects before glazing them 

A simple way to check ceramic dishes for cracks is to rest then on a
hard surface and lightly tap the rim with a hard object and listen
for its ring. If it is well made it should ring audibly and have a
second or so of sustain. If it is cracked, even a hairline crack, it
will may a dull thud like sound with no sustain. It’s not a bad idea
to check the thinner melting dishes periodically in this fashion, so
they don’t fail when melting and pour out all the grain.

Jason