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Heated Silver Become Concave


#1

Over the last few months I have been making some silver and 24k
pieces. The pieces are silver approximately 25-30mm in diameter,
1.5mm thick, with a piece of.1 mm thick 24k, with a centre cut out,
fused onto the surface. It takes me a few firings with a torch to get
the 24k completely attached and then I have been also texturing the
silver using the torch so I am doing multiple firings where the
surface of the silver is heating to the point of starting to melt. I
also tend to quench fairly hot, dull red, when doing this.

I have noticed that the pieces after a few firings are no longer
flat but start to take on a dish shape, very nice effect. So my
question is what is causing the silver to become concave? Is it the
differential in the expansion and contraction rates between the
silver and the 24k, in combination with the multiple heatings, that
is causing this, or is it caused by something else?

Thanks for any help in explaining what is going on.

Doug Frey


#2

I would has it a guess that your displacement theory is correct…I
am not an expert but i think its logical to assume that if the
expansion and contraction of the crystals have something to do with
it.

the next question that then comes up for me is that would this
happen in the same manner if the piece of metal was square instead of
round disc shaped? I am not sure…

Good question…


#3

Doug, I’m having trouble visualizing your piece so this is just
conjecture. When the surface is heated just to melting point yet the
main body is still solid, the surface is of course going to be
weaker than the solid core. This may dish it concave if there is
stress in the core that overcomes the surface strength. The quench
may add to this distortion. But since you like the results have fun
playing with it! You have pretty good torch control to be able to
fuse a .004" piece to something .060", or was that a typo?

Regards,
Neil


#4
I have noticed that the pieces after a few firings are no longer
flat but start to take on a dish shape, very nice effect. So my
question is what is causing the silver to become concave? Is it the
differential in the expansion and contraction rates between the
silver and the 24k, in combination with the multiple heatings, that
is causing this, or is it caused by something else? 

You are absolutely correct there is a large difference between the
gold and silver CTE (Coefficient of Thermal Expansion) so when you
lock the one side of the silver to the 24k the silver can no longer
freely contract and the strain that is caused by this causes the
silver to develop stress that forces it to change its curvature. I
have seen this phenomena actually break solder joints and diffusion
bonds. The strain can be so great that it can actually exceed the
tensile strength of the base material and it will not crack at the
bond but in the base material.

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts


360-756-6550


#5

I always see this happening when I’m annealing sterling/22k bimetal
elements for earrings. The strips are usually rolled down to 24-22ga
from thicker stock. I’ve always chalked it up to differing expansion
rates in the the two metals. No big deal…

Andy cooperman


#6

Jim Binnion’s post on this probably nails it. I’ll point out, too,
that if you are rolling out the metal (at first I assumed you
hadn’t), you need to make sure that you only roll it one way. If you
roll it “N-S”, and then roll it “E-W”, and you make a fairly large
piece, when you heat it up, you are guranteed to get a nice little
candy dish. It’s the grain you are forming as you roll - it needs to
be only one direction.

http://www.donivanandmaggiora.com


#7

Thanks Jim for confirming my hunch about what is causing my pieces
to become concave There’s a lesson here somewhere when differences in
the coeffecient of thermal expansion can cause such problems for some
projects but can be used in a positive way for other projects. I
guess its all about how you use it and knowing your materials.

Doug Frey


#8

Any, I’m curious, when this concave activity happens with Bi-metal
how do you keep the ‘fit’ that is necessary to do a proper solder
join? This has always been a problem for me when heating and trying
to solder Bi-metal to another piece.?


#9

On larger seams I rarely solder everything at once. I often tack a
portion, use this as a fulcrum and then either close and straighten
things up after pickling and resolder or close seams up while things
are hot. I’ve learned to do this making cylinders of 24g bimetal.

As soon as you start heating them the seam opens up. I am right
handed and hold the torch in my left, less dominant, hand which
leaves the right available to manipulate a pick, tweezers–whatever.
As I’m heating I apply a gentle closing pressure to the cylinder with
tweezers. This anneals the cylinder in place and I can then solder.

I do this with all kinds of things from rings to big hollow brooches
and from bronze to gold and steel. (I know of holloware makers who
employ this technique when applying “decks” to hollow forms.) I may
use a punch or probe to push with or any variety of altered tweezers.
Anyone who has taken a workshop from me has seen me do some variation
on this. I call it hot bending or annealing in place. Once the metal
gets hot enough, stresses begin to “bake out” or relieve themselves
and the metal responds to the pressure exerted on it from the
tweezers, etc. What you need to watch out for-- especially, I
imagine, with Argentum --is cracking or fracturing resulting from
over heating, spot heating, or overaggressive pressure.

Another thing occurs to me: I make bimetal earrings called "Whelks"
which are confetti like corkscrew shapes. Even if the strips that I
begin with cup a bit when I anneal them the cork screws themselves,
once they are made, rarely move. It has something to do with the
forming and the regularity of the stresses imparted, I imagine. It
really varies with the form.

I also suppose that most forms could be held in place with binding
wire, which would keep things alligned. I rarely ever use binding
wire. But regardless of what you use, I think that it’s important on
larger seams to solder a portion at a time so that adjustments can be
made. Soldering is one of those things that is a back and forth.

Andy Cooperman


#10
I'm curious, when this concave activity happens with Bi-metal how
do you keep the 'fit' that is necessary to do a proper solder join?
This has always been a problem for me when heating and trying to
solder Bi-metal to another piece.? 

The reason that the piece is collapsing or distorting may be that you
are overheating the metals when you are soldering. You are dealing
with two different melting temperatures and expanding temperatures in
the copper fine silver bi metal. You need to use an extra easy silver
soldering paste that melts at 1115 F and flows at 1205 F (#56). If
you have to bring the metals to too high a temperature to get the
solder to flow, the metals loose the shape created with the hammering
and under all that heat tend to collapse. You can also use an easy
paste solder that flows at 1310 F (#60) should you want to make more
than one join on your piece. After you have made one join and need to
make another, a great way to do this is to coat the first join with a
product called Stop-flow. This will keep the solder from flowing. If
you are going to do a bigger area, you may have to go to one of the
products that keeps your work cool, such as Kool Jewel. The Stop-flow
is also an excellent product to keep the solder from getting into
your patterned bi-metal, or any patterned metal. Simply brush on near
the seam and then the solder will not flow over the design. Remember,
every place you have put that product, the solder will not flow. The
reason I like this so much is that once you have finished the
soldering operation, Stop-flow will just wash off with regular soap
and water. It may need a boost from a toothbrush to get into the
little areas, but there is no big problem with removing it after you
have soldered. The yellow ochre that many are using to stop the
solder flow is so hard to remove and almost takes an act of congress
to get it off the piece. I get the Stop-flow at Mine Shaft in
Pompano, FL 800-654-3934 (no affiliation- just a good supplier)

Beth Katz Unique Solutions, Inc.
(http://www.myuniqesolutions.com
Paste Solder and Powder Solder for Metalsmiths and Jewelers


#11

To all that may be interested, from a UK goldsmith with 45 years
experience. An easy way to make precious metals perfectly flat after
mill rolling. This process is called plating flat by me and my
coleages, and to do this process you will need two pieces of 3mm
thick polished steel plate, of a larger dimension than the metal you
intend to flatten, then depending on size you will need some steel
screw “G” clamps. Over here in the UK we can buy steel called “ground
flat stock” which is perfect for his process. I will explain the
process as to flatten a piece of silver sheet that is warped.

  1. anneal the silver sheet

  2. when cool place the silver sheet between the two pieces of steel
    sheet, with the polished surfaces touching the silver.

  3. use the “G” clamps to grip the two steel plates tightly on the
    silver sheet.

  4. then with your forge torch heat the whole assembly of steel,
    clamps and silver, until it is a dull red.

  5. when the whole assembly is cool, take apart and you will have a
    flat piece of silver, which will not distort when re heated, if
    heated gently.

This is a technique taught to all silversmith and goldsmith
apprentices in the UK. I hope the is of use to someone
out there.

peace and good health to you all in orchidland James Miller FIPG, now
60 and still loving my trade after 45 years at the bench, although a
much slower worker now.

https://www.ganoksin.com/orchid/jmdesign.htm