Gold Plating

I have Gold plated the pans of flintlocks using the brush plating system sold by SPA Plating  (www.goldn.co.uk) with great success.   Steel makes a perfectly good substrate on which to plate gold directly without a barrier layer, the only caveat is that rust must be avoided by keeping surfaces very lightly protected by oil or a coating like Metalguard.   Spa plating used to have a very good handbook on plating but I couldn’t find it on the latest website, and the new instructions are less clear so I will put the .pdf at the end of this blog.  I have told them that the new website isn’t as informative!

Here are my hints for plating gold onto steel parts using the SPA plating brush method;-

1:  Polish the surface to a high brightness or the gold won’t look right.  You can get small Dremel type impregnated rubber polishing buffs in suitable sizes to do the inside of pans from jewelry parts suppliers ( see links) and small cloth buffing wheels and polishing compound to finish off the job.  Take your time to do a good job or you are wasting your expensive gold and any shortcomings will be emphasised by the gold plating!

2: Once you have polished the surface you must clean it until it is really chemically clean and make sure you don’t touch it after cleaning or you will undo your good work – best to wear latex gloves and make sure.   If you leave the steel around after polishing it and before cleaning and plating it some oxide will form on the surface and impede the process, so aim to go straight through with the job in one go.   First clean thoroughly in water with detergent – going over the whole surface with a clean wet tissue.  If in doubt use the SPA Aqaclean solution that is activated by electrolysis using the same equipment as the plating, and the same polarity *( -ve to workpiece,  +ve to electrode), but a somewhat higher voltage  (5 to 7 volts) with a small  brush on the platinum electrode. This cleaning will get rid of any oxide layers and result in a chemically clean surface – you should see tiny bubbles on the steel surface – rinse with clean water.

3: Mask off around the area you want to plate; the brush plating method doesn’t allow very precise control of the area to be plated as the liquid spreads out, so with a very fine artist’s brush use dilute shellac or any other varnish you have handy and paint areas you don’t want plated gold – for pans, mask the top edge, although this isn’t so important as it can be sanded off with 1000 grit paper on a lolly stick, but do mask around the sides of the pan as some gold will plate in unwanted places.

4: Once polished, cleaned and masked you can plate;   Wash out the brush in clean water and squeeze it out and slip it on the platinum or carbon electrode and connect the positive (red) wire to the electrode and a -ve (black, blue or green) wire to the object to be plated.  Connect the power source and set to around 4 to 4.5 volts ( 3 manganese alkali batteries ( pen cells) work well).  getting the voltage higher than about 5 Volts risks making the gold plating discolour and have black patches.   Tip a little of the gold plating solution into a small container – the cap of a small bottle for instance – and dip the brush in. Place the brush in the pan and move gently around – you should see the gold begin to plate onto the metal – you need to continue for at  least 10 times as long as it took for the initial gold haze to form – probably around 10 minutes, to get a reasonable thickness of gold.   If you are using the standard gold brush plating solution you may need to refresh the solution on the brush several times and it will take longer, I  prefer to use the fast solution, although it is more expensive it works out as the same price per gram of gold.

5:  Once you are satisfied that you have plated for long enough and have a good appearance, disconnect the power and rinse the parts and also rinse out the brush for future use.  Dry and remove the making varnish with meths if using shellac, or whatever will dissolve or remove your varnish.  clear up the remover and lightly oil the area.

Extra hints and tips;

I  use a proper low voltage power supply for plating – its the one I also use for electrolytic de-rusting – so that I can turn it on and off easily and adjust the voltage precisely, and also monitor the current flowing so that I can see how the process is going, and also estimate how much gold I am laying down.

The brush pads that SPA sells are rather large for our applications and thus waste precious gold solution.  They are also not cheap.  Since they are just pieces of furry fabric sewn together, I just buy a large one and cut it up and sew bits it into a size that fits the electrode and just does the job.  If you wash then out you can re-use them.

I use one of their proper platinum electrodes for gold and cleaning , but for silver plating I use a piece of gash pure silver.

What thickness do I get?  Science allows you to work out very accurately how much gold you deposit based on the current flowing and the length of time it flows for, although in this application the current varies a bit as you move the electrode around so you cant keep an exact check, but tis good enough for practical purposes.  The amount of gold deposited depends ONLY on the current and time and simple fixed factors that describe the electrochemistry of the process.  It is NOT necessary to understand these to estimate the amount of gold laid down, and, if you can estimate the plated area, to work out the thickness of the resulting plating.

Handy guide:  A small pan, say about 1 square centimeter, with a small brush that just about fits it will draw about  80 mA at 4 volts, and in 2 minutes should plate a layer of about 2 micron which is a reasonable layer  – in doing this you will use around 1/3 to 1/2 ml of solution but probably waste as much again because you shouldn’t put it back in the bottle.  If you use 1 ml, it will have cost you around £1.50 which is pretty cheap for a beautiful gold flashpan – just think what you can do with the other 24 ml in the bottle!

 

The scientific explanation;

The science works like this;  gold is carried through the solution as IONS that have an electric charge – these are atoms of gold that have lost 3 electrons – the greater the current and the longer the time, the more ions get carried to the surface to be plated.   Ions of gold in solution have lost 3 electrons each so they carry 3 units of charge each, and when one amp flows for 1 second it carries 6.2 x 10 ↑18  units of charge with it, so that is roughly 2.1 x 10↑ 18 atoms of gold.     Here are the relevant calculations;-

1 Coulomb ( C) = 1 amp flowing for 1 second or the equivalent, e.g.   100 mA flowing for 10 seconds

Ions / C	6.24E +18	ons/Coulomb
atoms/C	2.08E +18	ions/Coulomb/valency
grams/C	0.000680603	grams/Coulomb
grams/C	0.0006798918	from faraday’s equation – check
Cm^3/C	3.522E-005	cubic centimeters/C
cm/sqcm/C	3.523E-005	thickness /sq cm / C
microns/sqcm/C	0.352	Microns / sq cm / C
C per min	4.80	Coulombs per minute at 80 mA
u/min/sq cm	1.69	microns per minute at 80 mA
Grams / min	0.0033	Grams / minute at 80 mA
normal soln / min	0.408	ml of normal solution used per minute
fast soln/minute	0.272	ml of fast solution used per minute

 

 

 

 

Multiplying the number of atoms deposited by 1 amp for 1 second by the volume of one atom gives approximately  2 x 10 ↑18 divided by 2 x 10 ↑-23 0r 1 x 10↑ -5 cubic centimeters per amp second.    So an area of 1 square centimeter will get 10↑ -5 cm build per second, which is .01 microns for  1 amp for 1 second.  In practice a more realistic current for a small area of 1 sq cm would be 1/10 Amp,  so a build of .001 microns per second would be possible.  Thus a 16 minute (960 second) plating under these conditions would yield an approximately 1 micron layer, which is fine  – for a really heavy gold plate a 2.5 micron layer would be typical, so a plating time of about 50 minutes would be needed, although I doubt such thick layers are really needed in this application unless the gun is to be shot.    Bear in mind that those calculations are done for a 1 sq cm area assuming a current of about 100mA.  For a larger area the current flowing will depend on the area of  the brush and the resulting current flow.  Measuring it is the only sure way to know what you are getting!

 

the standard brush plating solution has 8 gm of gold per litre and the fast solution 12 grams/litre.

SPA old ;- plating manual -brush

Very technical stuff about gold plating;-

://www2.bren.ucsb.edu/~dturney/port/papers/Modern%20Electroplating/04.pdf