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 The Science, Facts and Fallacies of the Float Test

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DarthBerizing
Johnpaul Ragusa
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PostSubject: The Science, Facts and Fallacies of the Float Test   Fri Aug 25, 2017 10:14 am

Curious about the science behind the plastic? Wondering about repros at a molecular level? Collector/Scientist Lee Bullock has, and has put together a great article about the infamous "float test" and how science will play a role in battling repros!

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Staying Afloat in the Sea of Repro...  The Science, Facts and Fallacies of the Float Test

By – Lee Bullock BSc (Hons), MSc (Birmingham
)





Reproduction weapons for vintage Star Wars action figures have been the bane of collectors the world over for more than 20 years now.  Each year the profiteers and scam artists are getting better at their work with much of the production employing labour from China to mass produce these fakes.  

Over the years industrious and knowledgeable collectors have produced quick and simple tests to detect reproductions.  These include careful cataloguing in recent years of each of the mould families and variations of genuine accessories and compiling a reference library of fakes and in previous years the drop test and float test; the latter being used extensively in the hobby.  The main advantages to these were that they could be used in the “field” – you could go a toy show and be able to distinguish with reasonable confidence genuine from reproduction.

In this short article I will be looking at the science of the float test, what it actually tells us, its value in the hobby and its inherent limitations.

Eureka – that mouldy, old Greek guy!


The underlying science of the float test originates with Archimedes Principle.  Archimedes of Syracuse (c. 287 – c. 212 BC) was a Greek philosopher and mathematician who stated in his text On Floating Bodies

Any body wholly or partially immersed in a fluid experiences a buoyant or upthrust force equal to the weight of fluid displaced

This is a very important principle for our application because it links the propensity of an object to float or sink to its density and it’s that density that can help us gain information on what type of plastic we may be looking at.

So how does all this Archimedes Principle stuff work?

Well in explaining this there is bad news and good news.  The bad news is it’s a bit of physics to deal with; the good news is that it’s actually pretty simple...

It goes like this...  The net force of an object submersed in water is simply the difference in its weight and the buoyant/upthrust force exerted on the object by the water.  

Net force (sinking or floating) = weight of object – buoyancy force provided by the water

From this powerful principle you can relate the density of object to its mass on a balance and it apparent mass suspended in water.  Herein lies the point –

Water at 20°C has a density of 0.9982 grams per cubic centimetre (g/cc) any object that floats to the top surface of the water must have a density LESS than that at that temperature.

That’s it!  That’s all there is to understanding what we are seeing when we see accessories float or sink.  The floaters have a density of less than 0.9982g/cc and the sinkers have a density greater than 0.9982g/cc.  What we have done over the years is create a pass/fail criterion.

Tracking down the plastics – making sense of the float test...

Identifying the plastics used in vintage accessories by eye can be difficult without experience, but it turns out the float test helps.   A cursory search of the densities of common plastics yielded the following values

LLDPE (Low Density Polyethylene) = 0.915 - 0.925 g/cc

HDPE (High Density Polyethylene) = 0.97 g/cc

ABS (Acrylonitrile Butadiene Styrene– Injection grade) median value = 1.07 g/cc
Polyurethane resin (PU) – typical value = 1.10 g/cc

Poly Vinyl Chloride (unplasticised) = 1.45 g/cc

Poly Vinyl Chloride (plasticised) = 1.10 – 1.35 g/cc

Polypropylene = 0.946 g/cc

Given the density of water at 20°C is 0.9982 g/cc it can be seen that vintage accessories that float are either High Density Polyethylene HD PE or Polypropylene PP.  Now with some experience in polymers the best conclusion in the absence of fancy techniques like FTIR (Fourier Transform Infrared Spectroscopy) is most vintage accessories are made from High Density Polyethylene HDPE.

Reproductions that sink are clearly then made from another type of plastic, with the best candidate being ABS or cold cast Polyurethane resin.  So at this point what we have is a threshold point that eliminates plastics on the basis of their density.  However, as has been seen recently – many reproductions are now made to float!  Have repro con artists realised they should be making their wares out of Polyethylene and not Polyurethane?  More to come on this!

Reproductions in the balance...

At this point I thought it was worthwhile to get some careful density measurements of a sinking reproduction weapon – in this case a Lumat bow to try to ascertain what plastic it may be made from.

From previous work I had some baseline estimates for cold cast polyurethane resins – average density = 1.104 g/cc with relative standard deviation of 1.7%.

As the mass of the reproduction accessory was quite low I decided to use a four-figure balance to get the accuracy and precision needed to get a reliable density measurement.

Firstly, I carefully weighed the repro accessory dry in the top pan balance.  A mass of 0.3883g was generated.

Next, a beaker of water was filled and left to stand for one hour and then the temperature was measured with a thermocouple probe.  The temperature was 20⁰C and so the corresponding water density was 0.99823g/cc.
I then attached a cradle to the bottom fixture of the top pan balance with a thin, inextensible wire and suspended the pan of the cradle in the water.  The accessory was then carefully placed into the cradle and the suspended weight was measured after letting the system settle, the value for suspended weight was found to be 0.0199g.

With these numbers we can then determine the density of the reproduction accessory from Archimedes Principle - here is the last equation I promise!

Density of the repro accessory = (dry mass X density of water) / (dry mass – suspended mass)

So putting the numbers in...




The density of the repro Lumat bow was found to be 1.052g/cc which is clearly denser than water at 0.98823 g/cc and agrees well with the typical value for Polyurethane resin at 1.10 (±0.05) g/cc.

Final Thoughts…

The float test is a very useful technique for identifying Polyurethane resin reproductions in comparison to Polyethylene (HDPE) in the field, as the density of PU is markedly greater than water (PU sinks) and the density of HDPE is less than water (HDPE floats) – as such the technique relies on the density of the object relative to the density of water.  The float test only identifies and differentiates Polyurethane from HDPE; this therefore has primary use in identifying older generation reproductions.  

Modern reproductions have been recently identified that float – this clearly is evidence that reproduction manufacturers are starting to use HDPE like the original weapons and possibly recycling such original weapons to make these HDPE reproductions as it is a thermoplastic capable of being remelted for such a purpose.  The current crop of square-backed Leia blasters is likely to be an example of this new generation of reproductions.

In conclusion whilst the float test is useful it is not a standalone test and must be considered as part of an armoury of techniques and documentation of weapon details and versions by websites such as the Imperial Gunnery (http://www.imperialgunnery.com/) in order to reliably differentiate reproductions from original pieces.

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PostSubject: Re: The Science, Facts and Fallacies of the Float Test   Fri Aug 25, 2017 1:40 pm

This is OUTSTANDING. Thank you for posting this and thank you, Mr Bullock, for your work!

Dave C
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PostSubject: Re: The Science, Facts and Fallacies of the Float Test   Fri Aug 25, 2017 2:42 pm

Great work here Lee. Thanks for the excellent explanation.
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PostSubject: Re: The Science, Facts and Fallacies of the Float Test   Fri Aug 25, 2017 5:14 pm

Great post, thanks for sharing Lee!

While a lot of this goes way above my head, I appreciate the scientific explanation. I have seen mass density testing equipment used on gold. Is it accurate to say the density attained through injection molding can not be mimicked by cast methods?

With the exception of Poon, who was and probably still is injection molding Revalvit DT's, and as repros get better (as evidenced with the Leia blasters), I think density values for vintage weapons might be something that could continue to allow us to differentiate, but the issue is not everyone will have access to the proper tech/equipment to test and compare. For instance, we might already have the ability to measure using spectrometers, but not everyone can afford to have one kicking around on their desk or in their drawer.

Appreciate the effort you guys are putting into keeping VSW a safe trading place.
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PostSubject: Re: The Science, Facts and Fallacies of the Float Test   Sat Sep 02, 2017 12:37 pm

Great post. Required reading! Clearly proving one test method is not enough.

Multiple tests need to be used to confirm U.S. Morgan Silver Dollars:
1. Magnet Test. 2. Digital Scale. 3. Coin diameter. 4. Drop test. 5. Coin diagnostics--incorrect design, evidence of casting, no luster, etc.

For vintage Star Wars weapons, the recycling of vintage plastic is eye opening. I am not sure how the light test will compare between a vintage weapon and a re-molded weapon. Safe to say...no difference.

At this point (2017), the vintage weapons physical features are still the best way to weed out fakes. Fakes may have extra flashing and/or appear "ugly" upon close inspection. When the fake and the vintage weapon look and test identical, this will be sad day. Even Star Trek chroniton scans or C14 will not help.
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PostSubject: Re: The Science, Facts and Fallacies of the Float Test   Sat Sep 02, 2017 8:47 pm

Thanks for taking the time to put this together, great read and an eye opener not to rely solely on the float test.
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