I've got a lot of materials science study behind me, but never really embraced the core activities of metallurgy, namely smelting and casting. It's been on my mind for a while now, but I kept waving it off by reasoning that it would be far too expensive and dangerous to attempt on a residential scale. Recently I've come across a good number of videos and write-ups by people who've undertaken so-called backyard blacksmithing (by no means limited to iron) and the costs of their furnaces have been comparatively cheap. Some are definitely more professional looking than others, to be sure, but they're all a factor of 2-5 cheaper than the comparable commercial models. As for safety, I guess I qualify as "adult supervision", so as long as I keep all precautions in mind, everything will be fine.
So, with a couple aluminum projects in mind, I went about the task of comparing designs based on price, capability, aesthetic (it's silly, but everyone loves a sharp dressed forge), and ease of construction. There were some fun looking designs for electric arc and gas-powered furnaces, but I decided to limit my first foray to a forced-air and wood/charcoal affair. Capability was also an interesting beast, because I hadn't thought about the relative melting temperatures of various metals (excepting outliers like lead, mercury, and tungsten) in many years. I've thrown together a table for quick comparison below.
| Metal | Symbol(s) | Melt (C) | Melt (F) |
| Tin | Sn | 232 | 450 |
| Lead | Pb | 327 | 621 |
| Zinc | Zn | 419 | 786 |
| Aluminum | Al | 659 | 1218 |
| Magnesium | Mg | 670 | 1240 |
| Brass (85 Cu 15 Zn) | Cu+Zn | 920 | 1688 |
| Bronze (90 Cu 10 Sn) | Cu+Sn | 925 | 1197 |
| Silver | Ag | 961 | 1762 |
| Gold | Au | 1063 | 1946 |
| Copper | Cu | 1083 | 1981 |
| Cast Iron | C+Si+Mn+Fe | 1260 | 2300 |
| Manganese | Mn | 1260 | 2300 |
| Steel-High Carbon | Cr+Ni+Mn+C | 1353 | 2500 |
| Stainless Steel | Cr+Ni+Mn+C | 1363 | 2550 |
| Inconel | Ni+Cr+Fe | 1393 | 2540 |
| Silicon | Si | 1420 | 2588 |
| Medium Carbon | Cr+Ni+Mn+C | 1427 | 2600 |
| Nickel | Ni | 1452 | 2646 |
| Low Carbon | Cr+Ni+Mn+C | 1464 | 2700 |
| Iron | Fe | 1530 | 2786 |
| Chromium | Cr | 1615 | 3034 |
| Titanium | Ti | 1795 | 3263 |
| Tungsten | W | 3000 | 5432 |
Based on the numbers that others have posted, it looks like the design I selected (unless upgraded to gas or electric elements) will probably reach past 700-900 C depending on air flow and insulation. That being said, if everything works out with the first iteration, with an upgrade I should be able to work with metals up through copper without issue. I do plan on obtaining a thermocouple thermometer and logging the ramp rate and peak temperatures for a few configurations of fuel and air flow, just to see how far it can be pushed.
The general idea that I went with was a plaster and sand refractory cast inside of a steel bucket, with an air intake drilled in from the side. The initial build went quite quickly, allowing a full day for all the setting of the plaster parts. Over all, the materials for the furnace cost $51 at Home Depot, and left me with extra sand, plaster, and buckets. That doesn't include the hair dryer I already had on-hand that will become the blower on the air intake, but those can be found cheap.
The body and lid are already made, safety supplies and a good crucible are headed my way from Amazon! I'll post another update with the details of the build and, barring any unforeseen snags, photos from the first melt.