If you upgrade a 2h mace, you have a 5.26% chance for it to be a Furnace (source). So on average, one out of 20 upgraded rares may be a Furnace, and one out of 200 may be an ancient Furnace. As pointed out by Wudijo's math (https://www.youtube.com/watch?v=Rqsw9MtKB00) it's not smart to hope for an *ancient* version of rare items (such as a Furnace), so you're better off to upgrade rares until you get a Furnace and then re-roll it into an ancient one.
That being said: In most cases you don't even need or want an ancient Furnace, you'll just use the Furnace effect in your Cube and equip a well-rolled, easier to get item.
If you upgrade a 2h mace, you have a 5.26% chance for it to be a Furnace (source). So on average, one out of 20 upgraded rares may be a Furnace, and one out of 200 may be an ancient Furnace. As pointed out by Wudijo's math (https://www.youtube.com/watch?v=Rqsw9MtKB00) it's not smart to hope for an *ancient* version of rare items (such as a Furnace), so you're better off to upgrade rares until you get a Furnace and then re-roll it into an ancient one.
That being said: In most cases you don't even need or want an ancient Furnace, you'll just use the Furnace effect in your Cube and equip a well-rolled, easier to get item.