Making things feel heavy is cool because people feel like light things are cheap and worthless. But there are cheap high-density fillers.
One of the very cheap things Ecoquimica sells (see Material observations) is baryte. I was thinking I didn’t have any use for that, but it occurs to me that it might be useful as a high-density filler, with density theoretically 4.48 g/cc. And at Mohs 3–3.5 it should be easy to mill to finer granulometry. Baryte-filled silicone might be 3.4 g/cc and ought to be a bright, pure white, and much less resonant than pure or quartz-filled silicone. I see it’s also used for X-ray shielding, for making the first synthetic phosphor, Lapis Boloniensis (through carbothermal reduction to the anomalously water-soluble barium sulfide, which doesn’t melt until 2235°), and as a metal-casting mold release (melts at 1580°, decomposes at 1600°), as well as a source of barium for other materials like the remarkable and dangerous BaO₂. I’m wary of using it in reactions, though, because of barium heavy-metal toxicity.
For just adding weight and whiteness to things, zinc oxide (5.6 g/cc) might be superior, and is also interesting for oxychloride and phosphate uses, but from Ecoquimica it costs AR$5200/kg (US$29/kg) to baryte’s AR$83/kg (46¢/kg). Other vendors have it for lower prices like AR$1500/kg but nothing in the neighborhood of baryte. And if you just want density, you can get scrap lead, copper, and steel pretty cheap; a 30kg lead ingot is AR$13500 (AR$450/kg, US$75/30kg, US$2.50/kg), and I’m pretty sure the scrap metal guy around the corner pays something like AR$950/kg for copper, AR$700/kg for brass, and AR$100/kg for lead, all of which are more expensive than baryta (but denser). Steel is too cheap for him to even deal with.
For building machine tool frames, baryta’s vibration-damping and density properties might be highly desirable.
As a small dog nipped at my heels in the street, it occurred to me that maybe magnetite might be denser than zinc oxide and also cheaper, but as it turns out magnetite is only 5.17 g/cc, slightly less dense than zinc oxide. Related fillers that really are higher density include black cupric oxide (79.545 g/mol, 6.315 g/cc, boils at 2000°, -156 kJ/mol) and the safer but less air-stable red cuprous oxide (143.09 g/mol, 6.0 g/cc, boils at 1800°, -170 kJ/mol), both of which contain one oxygen and some copper (63.546 g/mol, 8.96 g/cc, boils at 2562°).
Also, any of those four oxides could serve as an oxidizer for self-propagating high-temperature synthesis (cf. SHS of magnesium phosphate); magnetite is 231.533 g/mol and -1120.89 kJ/mol (-280.223 kJ/mol O₁) and commonly used; zinc oxide is 81.406 g/mol, 5.606 g/cc, and a very tame -350.5 kJ/mol, and while it boils at 1974°, zinc metal boils at only 907°, which is why welding on galvanized metal is dangerous. In theory I ought to be able to make black cupric oxide from recycled copper for AR$900/kg × 63.546/79.545 ≈ AR$700/kg; copper dihydroxide (97.561 g/mol, 3.368 g/cc, -450 kJ/mol) dehydrates into cupric oxide at 80°.
On 02021-08-24 I walked by the neighborhood recycler (“COMPRO METALES, X MAYOR Y MENOR, Pago Mas!...”) to check on scrap prices. He’s offering AR$600/kg for brass and bronze (US$3.3/kg), AR$100/kg for aluminum (56¢/kg), AR$170/kg for lead (95¢/kg), and AR$900/kg for batteries (US$5.0/kg), which it turns out refers to lead-acid batteries, not disposable alkaline batteries (mostly manganese dioxide and zinc) — the 95¢/kg is the price for other lead, I guess things like tire weights, while the factory buys back the dead batteries for more than US$5/kg, presumably because of the high-purity lead content. He’s not buying copper at all today, it looks like; I wonder if that means he has scrap copper he can’t sell?
I've made copper hydroxide in the past by electrolysis, mixed, probably, with cupric chloride and the acetate, both of which are water-soluble, which should make separation easy. (Cuprous chloride is less water-soluble, but its Ksp is still 1.72 × 10⁻⁷, 13 orders of magnitude more than cupric hydroxide’s 2.20 × 10⁻²⁰, and at any rate it’s difficult to make.) If my objective were to make a lot of pure cupric oxide, I could use a plain vinegar electrolyte or a sulfate electrolyte to eliminate the chlorides. I bet Mina would appreciate the pigment, too.
So the points on the Pareto tradeoff curve for density to cost are something like:
More briefly:
22.65 g/cc Os (US$13000/kg), 19.3 g/cc W (US$30/kg), 15.6 g/cc WC ($???), 11.34 g/cc Pb (95c/kg), 7.9 g/cc Fe (21c/kg), 5.2 g/cc Fe3O4 (10c/kg), 2.6 g/cc SiO2 (3c/kg)
I feel like WC is probably cheaper than W, because Wikipedia says you can make it by heating WO₃ to 900° with graphite, and the USGS’s Mineral Commodity Summaries put WO₃ prices at US$148–US$270 per tonne for the last several years.
Mercury, litharge, minium, and cinnabar should probably be on the curve, too.
Not making the curve but still pretty cool, in part because of their colors:
I visited the recycler and bought a kg of copper wire from him, which cost AR$1200 (US$6.70/kg). He explained that lead-acid battery lead “is a different alloy” which is a reason for the higher price, also indicating the plastic case as another reason, but I think he sort of has those both backwards. Still, as a heavy filler, scrap lead with other impurities is probably close to the same density as pure battery lead.