Ol'Foundry

Foundry adds one block: the Foundry, a smelter with a few mechanics the vanilla furnace doesn't have. It burns solid fuel like a furnace, but the smelting speed depends on which fuel you use, it has a lava tank that speeds things up, and most ore recipes leave behind slag that you can do something with.

Fuel and lava

Any furnace fuel works (plus a couple that normally don't burn). Smelting speed depends on which one you use:

Most fuels — 1×
Coal / charcoal — 1.5×
Magma cream — 2×
Blaze rod — 3×

The Foundry has a 4,000 mB (4 bucket) lava tank. While it holds lava, smelting runs twice as fast — so coal becomes 3× and a blaze rod 6× — and the tank drains slowly while it works.

You can fill the tank three ways: right-click with a lava bucket, drop buckets into the bucket slot (they're consumed automatically), or pipe lava in with a fluid-transfer mod. Lava left in the tank stays with the block if you break it and place it again.

Slag

Smelting ores and metals produces slag as a byproduct. It has a few uses:

Reprocess it in the Foundry to recover nuggets — mostly iron, sometimes gold, with one extra if there's lava in the tank
Craft it into slag bricks
Burn it as fuel
Compost it

Other behaviour

The Foundry banks the experience from smelting and releases it when you take the output, the same way a furnace does.
A few recipes give more when lava is present — for example, ancient debris can drop extra netherite scrap.
Hoppers and pipes work per face: top inserts items to smelt, the sides take fuel and lava buckets, the bottom pulls out results and slag, and lava can be piped in from any side.