I've come across a few hundred pounds of pure lead.  It's so soft I can shave slivers off with an empty case and press a screwdriver into it.  

I've always used range scrap lead and it has worked great for me (big subsonic bullets). I cast and then powder coat to load large caliber subsonic ammo to shoot through a silencer.  

What's the easiest way to harden it up?  I see a bunch of various alloys on midway, but which one?  
I don't have a hardness tester or anything fancy and am not looking to spend tons of money and time.  "One pound of this with 20 pounds of pure lead" or similar is what I'm looking for.

I would at least  investigate selling/ trading the pure lead.You MIGHT find it's more valuable than a lead/ tin/antimony alloy.

I think there are calculators at this link to Rotometals.

I looked into it when I started casting because I thought my range scrap was too soft, but turns out it wasn’t. You can buy some of their “Superhard” to alloy with but I don’t know the ratio.

If you can trade or sell it to someone who wants dead soft you’ll probably be ahead of the game.

Bullet alloy information

I add Linotype from Rotometals.    50/50  pure/linotype for Hardball alloy (15 BHN).  Less linotype for target and non magnum loadings.   8 lbs pure 1 1/2 lbs linotype.    Antimony makes bullets larger in diameter as they drop form the mould & harder.

Get a lead hardness tester add Antimony and Tin till you get the hardness you want.

As others have said pure lead might be more valuable than the alloy.

ROTOMETALS

https://www.rotometals.com/super-hard-alloy-metal-ingot-5-pounds-30-antimony-70-lead/

keep some of it soft, in case you ever want to try black powder

Adding pure calcium.  I have Federals lead recipe somewhere. But calcium is the big one, I think sodium and phosphorus as well.

When I cast more I liked the Lyman no. 2 alloy.  Plumbers lead free solder is a good source for tin if I recall correctly.  

Also, the tin is used for its help in filling molds accurately (wetting) more so than hardening,  Antimony is used for it's for hardening properties.

http://www.lasc.us/Fryxell_Book_Contents.htm

This site has a great deal of info on casting, alloys Including tin, wheel weights & antimony, water quenching, etc.

I keep soft pure lead for BP, but, like others noted, add Linotype or Rotometal alloys.  Mostly, I use wheel weights for .357 & .44 magnum.

Best of luck—be warned, it’s addictive!

This exactly.

I mix 50/50 linotype and either pure lead or range lead (which is almost pure lead) to make hardball. I've tested hundreds of points of bulk indoor range lead and it was all about 99% lead, 1% tin (according to XRF).

Hardball is essentially a cheaper version of the famous Lyman #2. Tin is what is so expensive so making a hardball alloy saves you some cash but yields the same hardness as #2, just using less tin and thus less $$.

That's the alloy I use for virtually all of my casting, which mostly involves powder coating anyways.


When I'm mixing up different alloys I use this spreadsheet (will open as an excel document).

https://www.google.com/url?sa=t&source=web&rct=j&url=https://castbulletassoc.org/download/Alloy12.xls&ved=2ahUKEwi8rvPr1sTsAhVgmHIEHe-ECCIQFjABegQIDBAT&usg=AOvVaw0TkL58bgX-9rPhcxETRnjN

Any idea how hard range scrap lead might be?  It works great for me, I'd like to get this pure lead to a similar hardness.  
Edit: that link you posted showed range scrap to average hardness 10, that agrees with my experience based on interweb info.  

Not sure this lead is worth the time/effort/money for me when plated bullets are under 15cents...

If you're powder coating, what matters hardness?
I cast pure lead for my 388 BLK out and it mushrooms prefect

I'm not sure, I have only ever powder coated range scrap lead.  Does hardness even matter when powder coating?  

All the info online says pure lead is good for around 800-1000fps.  I imagine water quenching and powder coating increase that allowable velocity?  
Looks like you can mix 1lb of Linotype lead to 12lbs of pure lead to approximate range scrap.

I prefer adding some tin for mold fillout more than anything. There's no doubt that adding tin into lead will promote better bullets. I managed to get a hell of a deal on about 500lbs of linotype, so it's easy to just do a 50/50 mix for hardball. Considering I shoot a lot of 300 blackout, which could benefit from harder alloy at higher velocities, it's easiest to just cast everything with the hardball. Great fillout, shoots well in pistol and rifle, and easy to make. Perfect combo for me.

I've used pure range scrap and it works just fine for pistol. Haven't tried it with rifle though.

Powder coating does not replace copper jackets. Adding a power coating will not make your lead a whole lot harder, so I would still abide by the general rules of hardness vs. velocity. If you don't believe me, powder coat a bullet and then smash that bullet and an uncoated bullet with a hammer. You'll notice there is not much difference.

Powder coating is beneficial because it prevents gas erosion of the lead as gas blows by the bullet when shot. That's actually what causes leading, not the mechanical action of the lead scraping against the lands. That's also the principle behind how gas checks work, they block gas from passing by the lead. Lube was previously used in lube grooves because as the gas passed by the lube groove it would essentially lube the lead/lands and prevent lead from melting/sticking to the barrel.

So use powder coating as a replacement for gas checks to prevent leading, but do not use it as a means for making an alloy harder.

No matter the lead hardness it seems that powder coat could prevent leading from gas blow-by and from the base of the bullet.  Question is how much?  

Again, leading is my only concern for loading big-bore subsonic ammo.

Here's a link to an Excel-based alloy calculator. I've found it to be quite accurate.

While you're at the Cast Boolit forum, check the classifieds for linotype, it's not all that expensive.