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[Firnagzen]

So I'm going to embark on my first obsidian generator. Just a couple of questions to help me get this right:

Does it make a difference whether I add water to magma or water to magma?

How much water does a 7/7 magma tile evaporate before it turns to obsidian? I'd like to calculate this one precisely so I don't have excess water or magma slopping everywhere.

[Stargrasper]

I've never done this, but...the wiki entry on Magma is your friend.  Answers all your questions, as near as I can tell.

[Mechanical Yeti]

It shouldn't matter which goes into which.

Water and magma cancel each other out completely. So 7/7 water is canceled out by 7/7 magma. No more or less.

[Grumman]

Magma will evaporate a 1/7 tile of water without producing obsidian. Apart from that, any quantity of magma and any quantity of water in a single tile will create obsidian. For my current obsidian farm I use a pair of 3*3 rooms to measure the amount of water and obsidian to drop into the mould.

[Albedo]

There is a definitive thread on magma farming here:

http://www.bay12games.com/forum/index.php?topic=26805.0

[Firnagzen]

Ah, excellent. Thanks.

[yuhhaur]

I used a 7x9 (x7) = water drop to magma. the magma reservoir is also 7x7. the magma farm is 13x13 so I can have 2/7 magma, pour >2/7 water onto it. the 7x7 water reservoir has 2 floodgate output to 2 obsidian farms. It combines the water and magma effectively leaving 1 tile of excess water to evaporate. I purposely did this to avoid inadequate water to cover all the magma.

[Albedo]

Firnz- np.

The most important concept is that, if you are using a retracting bridge to drop the water (or the magma), it must be two z-levels up from the future obsidian.  If it's just one up, it interferes with channeling out the finished product.

For the "perfect amount" of water, use a two-stage system.  Your retractable bridge(s) will need about 2/7 of water - a little less if you want faster evaporation, but too little will leave un-transformed magma, a bad thing.

Take the size of the bridge(s), and multiply their size by 2 (or 1.9 or 1.5 or whatever) to determine the total water you need.  Dig a "feeding" reservoir of that amount/7 above that.  Put a hatch on the drainage end and a drawbridge on the inflow end, so when the hatch is open the floodgate is closed, and vice versa.

So if your bridge is 10x9, that's 90.  90/2 = 45, and 45/7 = about 6.5 - so I'd dig it 6 tiles big. About 42 water* will fall onto your retractable bridge each time you cycle that - when the water pours out, nothing pours in; when the outflow is closed the inflow opens, and it fills 100%, no more no less.

(* less some left over as 1/7 pool.  In theory, if every tile of the 6-large reserve kept 1/7 water except the hatch, you'd only get 40.  For higher accuracy, you could have a 2nd retractable bridge above your main one.)

If/as you want to refine the amount of water, you can make the reservoir 1 tile larger (7/7 more), or put a wall in for 1 tile (7/7 less) smaller, changing the total amount of water by about 6-7 each time (depending on whether 1/7 is left or not.)

[Skorpion]

Excess water/magma is dwarvenly. Also, it lets you correct screwups.

I mean, I flooded a chunk of my fort with magma while expanding the forges. Since I'd already made plans for an obsidian farm there, it was a simple matter to just flood the area with a whole load of water, then dig out the channel to make another tower-cap farm with the low water remaining there.

As for excesses, my prior fort had about 10 tiles of 7/7 magma in the farm, with a pair of pumps left on for a good while, pumping sourced water from the brook.
I eventually had to build an aqueduct into an expanded murky pool to cope with the runoff.