Getting the Lead Out (Couldn't Resist :-))

This post reports on some investigations I did into finding a replacement for lead as ballast in small model yachts. Lead is bad news, especially for the young. It must be ingested in order to poison you or yours, but that can happen through inadvertent transfer from hands or clothing. Even if you are careful about your hygiene, the act of melting lead generates lead fumes which you can easily inhale. In any kind of a school setting casting lead is pretty much out of the question these days. Trying to explain the intricacies of lead exposure to a nontechnical parent whose only knowledge of the subject comes from news reports of toy recalls is not a task that I, for one, would look forward to. It’s much easier to be able to assert with confidence that no exposure is possible.

First, some numbers: lead is generally described as having a density of 11 grams per cc. This is an ideal number; the sort of scrap lead we usually deal with is more like 9.5-10.5 g/cc depending on what it is alloyed with. The closest solids that are economically within reach are the copper alloys, which run roughly from 8 to 8.5 g/cc. The disadvantage here is the effort required to shape the material, especially if a bulb keel is desired. I designed Yankee III to be ballasted by bricks of 1/2 x 1/4 in scrap copper bus bar, which at the time of writing was cheaply available. The price has risen sharply since then.

The next thing I looked at was using a rubber mold to “cold cast” a bulb from resin loaded with some heavy but safe metal. In the case of the student program I’m planning I really want to have the students cast their own ballast so they can go through the process of weighing the finished boat and calculating how much it would take to get her to float on her lines. This eliminates raw lead shot or “geezer cast” solid bulbs as candidates.

After some searching I found three materials that are readily (in the US, at least) available on the net: steel shot, copper plated lead shot, and bronze powder. The shot runs $2-3 a pound and the powder about $12 a pound. I weighed 50 cc measuring cup with various mixes. The inhomogeneous mixes were done by the Martini Method (shaking the ingredients together in a jar), and this is what I got for densities:

#6 Steel Shot (0.11 in/2.8mm dia): 5.5 g/cc
#7 1/2 copper plated lead shot (0.09in/2.3 mm dia): 7.1 g/cc
Steel and plated lead shot mixed: 6.6 g/cc
Plated lead shot and bronze powder mixed: 7.3 g/cc

The weighing was done dry; the addition of resin will reduce the density to the degree that the casting is “overloaded” with more resin than is absolutely necessary to bind the material. The result for the shot/powder mixture is promising. A bulb made from this material would only need to be 10-15% larger in linear dimension than a one cast from solid lead.

As a final experiment I attempted to see how much bronze powder could be packed into the plated lead shot by using my scroll saw base as a shake table. The resulting density was 8.5 g/cc, right up there with solid copper alloys. This was a pretty solid mass, and I doubt that enough resin could penetrate it to make a strong solid; so the preliminary result is that it’s only practical for filling bulb shells such as those Nigel makes, using heavily thinned resin as a binder.

So, in the interest of spreading the boundaries of Footydom (“The Sun Never Sets on the Footy Fleet?”) it would be nice if some supplier would package plated shot and powder in, say, 200 gram kits and made them available to those who would rather not hassle with lead. Cold casting a filler into a pre-made shell involves some cost in frontal and wetted area, but I think that in most cases this will be more than repaid by the ability to get such a small boat to float precisely on her lines.



Two things:

  1. Why do we have to fill ‘shells’ that become part of the finished structure? Can they not be removed like any other casting mould, thereby increasing the 0verall density of the bulb.

  2. From past experience (in UK and 25 years ago), there is, as you say, a tremendous range in densites. The important buzz-words are ‘scrap lead’ and lead scrap’. Scrap lead is recycled lead and typically has all sorts of impurities in it that educe its density. It is available as ingots, sheet, foil, etc, produced by melting down ‘lead scrap’.

Lead srcap is individual pieces of scrap from one or more sources. Depending pn the source it may be very close to pure lead (19t century plumbing) or very highly contaminated (car batteries). Given the tiny quantitities required for a Footy bulb, if you can get your lead close to source (i.e. before melting down), it is possible to hand pick high density pieces. Making friends with your happy local plumber works wonders.

Sorry, wasn’t clear. I was referring to the tight packed 8.5 g/cc mixture, which I doubt would absorb enough resin to be sturdy on its own.



I haven’t ttied this, but a very switched on acquaintance of mine suggests that adding around 15% by volume of alcohol (he says 'any alchol, but let’s assume just ethanol or metanol) to epoxide rssin reduces its viscocity hugely. It also exxtends the time it takes to go off substanially. We were talng about this in the context of wood/epoxide saturation, but resin perfusion of a powder matrix seems an equally appropriate application.

I am told that the strength of the resijn is not substantially impaired.

I recently cast a Footy bulb in “Aquabond”, a new solder from Kester, which is lead-free and intended for use on water pipes intended for drinking water.
I live in London, Ontario, Canada, a city of 350,000, which has found to its horror that some of the old supply pipes and many of the modern fixtures are leaking lead into the drinking water. The water in many schools has tested up to 100 times the “accepted” limit, and many pipes and fixtures (taps and drinking fountains) will need to be replaced, as well as many supply pipes under the streets. The problem may well be low pH, which leaches lead out of previously harmless solder joints and plumbing castings.
This solder has a sp. gr. of 7.0 (approx) and is mostly antimony. It is fully machinable, on a lathe, for instance, and can be drilled with ordinary wood drills.( Sorry, 97% tin,2% copper, 0.8% antimony, 0.2% silver.

(quote=Angus;42116)I haven’t tried this, but a very switched on acquaintance of mine suggests that adding around 15% by volume of alcohol (he says any alcohol, but let’s assume just ethanol or methanol) to epoxy resin reduces its viscosity hugely. This is true. Acetone and lacquer thinner will also reduce viscosity. 15% seems like a huge amount, but then I’ve mixed by eye/experience - not be weight or volume. Subjecting the resin and hardener to a bath in hot water for about 10 minutes before use (inside their respective cans, of course) will serve to “thin” the epoxy as well.

It also extends the time it takes to go off substantially. Alcohol will - but only slightly. Depending on what product you are mixing in - will determine the extended time. The thinners with higher flash points and fast evaporation will not extend time a significant amount. It may seem that way since a larger area can be brushed/covered by the thinner mix. Epoxy cures by a chemical reaction, and while thinning will extend that reaction time, since both resin AND hardener are mixed equally, all you have is a “thinner” mixture.

We were talking about this in the context of wood/epoxy saturation, but resin perfusion of a powder matrix seems an equally appropriate application.

I am told that the strength of the resin is not substantially impaired. Most epoxy companies will probably argue that point to some degree - but one must remember, it’s their product and their liability that is at stake. I have used thinned epoxy on many projects - usually as a saturation first - followed by a coating of “neat” resin - or resin that has been thickened with fillers - depending on application. I found doing peel tests - thinned epoxy tends to be and remain a bit more rubbery compared to epoxy mixed with proper amounts of resin and hardener. Even this remains “soft” in comparison to polyester resins which seem (and feel) harder and more brittle - a personal opinion here.

All of the major epoxy formulations are being changed daily - and many have retained their adhesive qualities and reduced cure times. Others can now be used underwater or in wet areas. Others still are improving the product viscosity by different formulations or choices of hardeners. With rare exceptions, most using epoxy prefer the thicker formulas as it aids in vertical and overhead applications - but sure doesn’t help balsa boat builders reduce weight very much when the final product is so thick. :smiley:

I don’t think I would fool with recommended mixtures on any projects where life is at stake, but on model boats - why not give it a try? :scared:

for the millionth time :stuck_out_tongue:
i produce shell halves, for any one who dosen’t want to melt metals.
i fill mine with resin and buck shot.

to get my weight correct, i fill one half with shot, weigh it, and add resin to bind.
then subtract that total half weight, from my overall target weight.
then i do the next half

Thanks for bringing this up, as I forgot to mention the lead-free pewters AKA “britannia metal.” Current prices from:

are about $12.00 per lb, which IIRC is considerably less than they were when last I looked. These metals have the advantage that at a melting point of 466 deg (F) they can be cast in room temperature vulcanizing (RTV) rubber molds.