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sarinox
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[*] posted on 26-12-2021 at 03:33
A possible fraud!


Hello all, I hope you all are doing well,

Recently I had a 3D model printed with wax (Projet wax printer).

Later I gave the wax piece to a person who casts wax models into Silver.

This person allegedly uses Sterling Silver, and as we know Sterling Silver should have a Density of 10.37 (gr/cc).

The computer could calculate the exact volume of the piece therefore we could predict that the produced piece should weigh around 16.8 grams. However, the actual produced piece weighs 14.5 grams!

I had checked the measurements of the produced piece it is not different from the measurements of the 3D model produced in 3D modelling software.

Now I wonder why the produced piece lacks around 2.3 grams!

I thought maybe because the Stirling Silver is heated up beyond its melting point in the casting process its density would decrease & this assumption is confirmed by the data found about Silver's density beyond its melting point (notice I was able to find the density of melted Silver and not the density of melted Sterling Silver which is an alloy of Silver!)

And most importantly if we assume the density of Sterling Silver decreases during the casting process ( Which I believe this is what actually happens) then the produced piece should show contractions in its dimensions or we should be able to discover cavities in the product; which we were not able to find any of them in the visuall examination!

If you know about thermodynamics of mixtures then possibly you can find the density of melted Sterling Silver and maybe you can find an explanation for the missing 2.3 grams!

For now I think the Silver alloy used to cast the piece was not Sterling Silver!

Sterling Silver composition: 92.5 percent Silver + 7.5 percent Copper.

I'd be happy to know your opinions on this issue.

I think the density of the alloy used to cast the current piece is around 9.3 gr/cc or 9.4 gr/cc I don't know any commercial alloy of Silver with this density do u have any guesses?

Best,
Sarinox







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unionised
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[*] posted on 26-12-2021 at 05:06


https://www.youtube.com/watch?v=UQ97jkVnX0A
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sarinox
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[*] posted on 26-12-2021 at 05:46


Hello, Unionised,

The youtube video link u posted is it a song? ( Was it a mistake?)

Best,
Sarinox
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Fulmen
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[*] posted on 26-12-2021 at 06:02


Start by measuring the actual volume (by weighing it in water), casting usually involves some shrinkage.



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Amos
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[*] posted on 26-12-2021 at 06:21


Quote: Originally posted by sarinox  
Hello, Unionised,

The youtube video link u posted is it a song? ( Was it a mistake?)

Best,
Sarinox


It's intended as a statement that there are trapped air bubbles in the cast, reducing density.
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sarinox
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[*] posted on 26-12-2021 at 07:10


To Unionised

Wooow, :-)

A smart way of answering! :-)



[Edited on 26-12-2021 by sarinox]
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sarinox
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[*] posted on 26-12-2021 at 07:17


Quote: Originally posted by Fulmen  
Start by measuring the actual volume (by weighing it in water), casting usually involves some shrinkage.


Well, right now I don't have access to the apparatus which are needed to do that.

However, I measured different parts of the piece with a calliper and it reads the exact same measures as the computer model shows!



[Edited on 26-12-2021 by sarinox]

20211226_185325[1].jpg - 2.7MB

[Edited on 26-12-2021 by sarinox]

3D_Model.jpg - 115kB
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sarinox
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[*] posted on 26-12-2021 at 08:06


As u can see in the above post, the computer says the dimensions of the cube should be 18.4 mm but the Calliper reads 17.96 mm. (I know this is because of the curvature of the edges of the 3D model as the computer measures the farthest vertices from each other so it reads 18.4 instead of 18) but let's assume in each side we have 0.44 mm contraction, therefore, we should lack a cube of Volume 0.085184 mm^3. which does not justify the 2.3 grams lack of weight in the produced piece.

I am confused! the model has thin walls as u see and I see no cavities! at least they are not visible on the surface.
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Fulmen
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[*] posted on 26-12-2021 at 08:14


Your math is wrong, you need to scale the entire part: 16.8g * 17.96^3 / 18.4^3= 15.6g

So the shrinkage can explain at least half the discrepancy.




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sarinox
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[*] posted on 26-12-2021 at 10:49


Hi Fulmen,

Thank you for that! I got your point somehow ( well 18.4 ^3 gives us 16.8 therefore how much will 17.96 ^3 would give us! ) I got it!

So I got: 18.4^3 - (0.44^3) is not equal to (17.96^3) or ((18.4 - 0.44)^3)
Sorry, my bad!

However, there is still 1.1 gm missing! ( I hope I can find an explanation for it too) :-)
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Fulmen
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[*] posted on 26-12-2021 at 12:11


A volume measurement would be most helpful.



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[*] posted on 26-12-2021 at 22:12


I checked the expected shrinkage of cast silver at two web sites. One gave a figure of between 1% to 6% and the other 4.6% (for a cast ring). In mass that would approximately correspond to 3% to 18% and 13.8%.

The OP's mass difference of 13.7% is probably reasonable. Check your bill to see if you have been charged for the mass of the silver and if it corresponds to your measurement.

[Edited on 12/27/2021 by wg48temp9]




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[*] posted on 27-12-2021 at 02:04


Considering the pure silver price is around 70c per g, it is likely that most customers would not bother themselves with that small of a difference, especially when we consider that the item appears to be a fancy looking jewel, not a scientific calibration standard. Considering shrinkage and other variables in such a small item, the tolerance seems reasonable. Usually when people order castings, they are sold as a product, not as a mass of unit, so the precise weight can have quite the variation. Castings are notorious of having different than calculated weights and volumes due to shrinkage, mold variables including type, material, porosity, surface quality, post processing, temperature of each step and so on. They produce very uniform products in mass production, once everything is dialed in.

If it was gold, it could be different due to the 100-fold price. However, it is likely that the cast would be supplied either "as is" or with a tare weight to alleviate any accusations.
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[*] posted on 27-12-2021 at 02:39


Silver is notorious for getting bubbles in it when you cast it.
When it is molten it absorbs oxygen from the air and that bubbles out when it freezes.


Quote: Originally posted by sarinox  

Well, right now I don't have access to the apparatus which are needed to do that.

I suspect you have; all you need is a balance, a beaker of water and a piece of thread.
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AJKOER
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[*] posted on 27-12-2021 at 10:10


If one pours sugar in a cup to measure and pours out, there is some adhesion based loss.

Also, if one is less meticulous in their work (sloppy, spillage) expect even less. Bargaining for a lower production cost invites a lower standard of care.

There is a reason as to why people wants to see customers' ratings.

But in the end, you have to decide as to whether paying the higher price is worth the difference in quality.

[Edited on 27-12-2021 by AJKOER]
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[*] posted on 27-12-2021 at 11:37


How did the person pouring the silver charge you? By the volume of your piece?

How much did he charge a gram? Silver goes for about $0.73 a gram.
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[*] posted on 27-12-2021 at 12:55


To pull off such a scam, the caster would have to somehow homogeneously shrink a piece of 3D printed material that the customer suppplies (!), saving maybe $1-$2 of material.
Why would they? I'm sure the total cost of the piece is completely dominated by the cost of labor, not raw materials. They could also simply add $1-$2 to the final bill and 99% of customers would not care.

If you are really determined to pursue this, I suggest you make another order: This time of a simple geometric shape, eg. a cube. Then determine dimensions, volume, density and perform a silver assay.
If the silver content is different from what was specified, then perhaps you can call 'fraud'.



[Edited on 27-12-2021 by phlogiston]




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[*] posted on 31-12-2021 at 08:11


you can take it to a pawn shop that has an xrf and get an appraisal.
they will tell you the alloy and weight.

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[*] posted on 31-12-2021 at 13:19


@phlogiston: I believe he suspected an inferior (lower density) alloy was used, and that could indeed constitute fraud. personally I don't think one should throw such claims around lightly....



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[*] posted on 1-1-2022 at 05:13


So I had a look round on the web and found this which tells me the density of some copper/ silver alloys
https://www.thesscogroup.com/products/industrial-products/si...
And I also have the density of the pure metals so I can draw a graph.

From the graph I can estimate the copper content needed to bring the density down to 9.4 or 9.3 as about 70 to 75% copper.
That's quite a lot of copper- I wondered what colour that alloy would be.
So I made some.
I melted 0.25 grams of Ag and 0.75g of Cu together.
The resulting alloy is distinctly yellow.
So, I doubt that the work you received from your metal casting works has enough copper in the silver to make the alloy with that low a density. You would see the colour.
It would, I guess, be possible to cast the metal undersize in copper then plate it with silver but that's so much work it wouldn't be worth the cost of the silver saved.
It's fair to say that I struggled to get that alloy to melt properly so, if there's anyone out there with better skills and kit who could repeat the test I'd be grateful.

And, of course, copper isn't the only possible alloying element.


AgCu density.png - 9kB
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[*] posted on 6-1-2022 at 08:01


2.3 grams and you call this a fraud ?
Why not call it grand larceny or let's make a movie about it. George Clooney is available.




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