Inlaying Platinum

[mitch]

A few weeks ago when I posted pics of an 18kt rose gold Lake folder (http://www.engraverscafe.com/showthread.php?9926-18kt-Gold-Lake-Platinum-Inlaid.....), a couple people asked about inlaying platinum. My initial response was just to say that I use pure platinum, as opposed to 95/5 or 90/10 jewelry alloys, and that it works pretty much like 24kt gold. After thinking about it a bit, I thought more should be said on the topic.

First is that all-important, pure, 100% (or 99.99999... or whatever), platinum. It's pretty soft stuff- not quite 24kt, but not bad. However, the main difference lies in its lesser ductility, or malleability, than any relative scale of hardness/softness.

As most of you probably have noticed, 24kt gold is one of the most ductile substances known to science, and certainly THE most known to engravers. I do all my inlays with sheet gold (30g B&S, 0.010" for flush inlays), and by employing certain techniques in prepping the cavity (more on this in a minute) i can be pretty lazy- within limits, of course- snipping out the piece to inlay. with a properly shaped cavity of the right depth, 'good enough for government work' is also usually 'good enough for inlaying 24kt gold', knowing that i can make the metal flow anywhere i want it. such is not the case with platinum. you must be much more accurate in cutting out the inlay. with gold, if you have more than you need off in another zipcode you can, with judicious application of the right shape punches, build & push a substantial wave of metal over where you need it to fill out the edge of the cavity. this trick is much less effective with platinum. it'll work, but only over much shorter distances with much smaller quantities of precious metal. any attempts to overdo this process tends to make the metal surface gall & flake as it work-hardens.

Getting back to cutting the cavity, with gold i routinely leave the middle a bit shallow (0.002"-.003"?) while making a more nearly full depth trench around the perimeter. this allows me great latitude in applying the aforementioned technique of moving metal where i need it, knowing i've got plenty pretty much anywhere away from the edges. also, gold will easily flow down into this deeper trench & into the undercuts- platinum is less amenable in this regard. so in addition to having to cut the inlay to fit the cavity more precisely, i cut the overall cavity closer to full depth as well, thus further reducing my margin of error. at the very least, i gently slope the raised middle area down into the edge trench so that the platinum has an easier transition into the undercut.

there are a few factors in determining how much of each method to employ. in a big, solid, sturdy steel gun part i'd be inclined to treat platinum much like gold- a little deeper cavity and a fairly accurately cut inlay, but i can still whack it pretty hard to do what i want. however, on a precision-made folding knife with an 18kt rose gold frame, i want to be as gentle as possible. this meant near-full depth cavities, generous undercuts (but not too generous, as i don't have a lot of metal to spare), and very exacting inlay pieces. also any degree of curvature to the part means everything has to be done even more particularly to avoid metal creeping out of the cavity as it's being set- just like gold, only more so with plat.

the one thing i really like about inlaying platinum? if you're using a common propane/air torch, it can be annealed with wild abandon! set it on a soldering pad, turn it bright pink if you want and even the finest wire or detail points are in no danger of melting or oxidizing.

 

[Sam]

Thanks for the great info, Mitch. I've added your post to the Tips Archive

 

[Dave London]

Thanks Mitch
Realy good info

 

[Ed Westerly]

Very cogent and lucid! I enjoyed reading your technique. :biggrin: Thank you for sharing.

 

[SamW]

Platinum takes 2100 degrees to anneal and about 3000 to melt. It does work harden fast, like the first lick. I have only inlayed platinum in wire form but have found that if I get everything set and ready and set the platinum with the first hit I am OK. Otherwise starting over is sometimes (always) needed as the edge of the steel can collapes slightly making setting almost impossible and redoing the undercuts required. Looking at the white hot platinum when annealing is, I am told, not good for the eyes.

 

[Kevin P.]

“i routinely leave the middle a bit shallow (0.002"-.003"?) while making a more nearly full depth trench around the perimeter.
i cut the overall cavity closer to full depth”

Perhaps you’ve already answered this; what is ‘full depth’? If you’re inlaying 0.10 sheet is anything less full depth cavity?

How do you measure that .002”-003”? A go/no -go gage?

Good post Mitch, your exposition is quite good, meaning clear.

As T O Sam mentioned eye safety: there are special glasses that protect one’s eyes. You can do serious damage without protection.
Kevin

 

[mitch]

Kevin-

assuming the use of .010" thick metal and maybe .001"+ as a safe margin of error (i try to never cut the cavity too deep anywhere- it's a pain to correct/fix), i cut the 'full depth' perimeter trench approx. .009"ish or so. this means the middle of the field should be at least .006" to maybe .0075"- on the deeper side for bigger inlays, so they're not too weak/thin, smaller inlays are not that critical. also thicker still if there's to be any significant engraving/shading in the precious metal- you don't want to cut thru it. (note: while i don't use one, i can see the value of 120 degree gravers in this situation. they give the line width/weight you need at much less depth.)

of course, the tricky part is how to consistently cut those cavities to the right depths. the short answer is 'practice, practice, practice'. after about a thousand inlays you just learn what looks about right. the key- for me, anyway- is to use a consistent technique cutting the cavities. i find that using a 90 degree square* graver (i like well-dubbed carbide because it's just hogging out metal) to cut rows of fairly heavy/deep slightly overlapping lines- so the original metal surface is completely removed, followed by a flat graver to remove the resulting ridges, leaves a cavity about .007"+ or so. obviously, this technique is heavily dependent on years of experience cutting those grooves and shaving the ridges off, so it turns out about the same every time, but i'm sure you'll get hang of of it pretty quickly. it also ensures a pretty consistent depth across the entire cavity, without the need for a reference surface.

*rather than trying to measure depth of a cut, which can be tricky to do accurately, especially over a curved surface, keep in mind that a 90 degree square cuts a groove about twice as wide as it is deep. it's not too difficult to measure the width of a line/groove against a very fine scale or caliper under a microscope, then figure it's roughly half that deep.

btw- i don't really get that carried away annealing platinum. i get it a nice bright cherry for a few seconds and leave it at that. but it's nice knowing that it's virtually impossible to melt it & ruin a half hour of work getting an inlay exactly right.

 

[KCSteve]

*rather than trying to measure depth of a cut, which can be tricky to do accurately, especially over a curved surface, keep in mind that a 90 degree square cuts a groove about twice as wide as it is deep. it's not too difficult to measure the width of a line/groove against a very fine scale or caliper under a microscope, then figure it's roughly half that deep.

Nice tip - thanks!