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[Footkerchief]

Mirror should be treated as a construction, made from a bar of silver using metal-smith task.
The mirror need to be constructed outside where the light hits.
You can use the "q" command on mirror to get a cursor that allow you to redirect that beam to a specific location with some limitations (45 degree blindspot below it).  Treat light's path similar to a bolt (will be stopped by obstruction).  Redirecting will be a task and needs a dwarf to set their focal point.
Each mirror will generate heat at the point it's beam hits (increase temperature).
Get enough mirror all pointing to the same spot can make it hot enough to catch on fire, burn noble, or goblin.  Even more mirror will make the tile like a magma (allow furnace operation).
So, what do you guys think?

Since the thread has come here: the idea of mirrors as constructed objects with a specific orientation came up in my old Lighting Arc discussion thread, and they would be quite straightforward to implement with even a rudimentary lighting simulation.  It would also be pretty straightforward to link a tile's lightedness to its temperature.

As for the tech level considerations: it's completely indisputable that a large mirror array was well within the conceptual and industrial grasp of a bunch of Greeks circa 200 BC, whether or not the mirrors managed to achieve their purpose.

[cparax]

For one, smelting isn't melting/alloying, and I think you're confusing these terms. To smelt iron from iron ore, you still need a carbon source, for instance.

Which presumably would be introduced the same way as it is in magma smelters. Metallurgy isn't really my area so I'm not sure what way that is, but it has to be possible somehow.

Also, this sort of solar array heats things up at a focal point. A big vat of iron would probably have trouble maintaining temperature if you're only heating up a very small portion of it, probably.

This would be a problem with metals that aren't particularly good conductors, but keep constant heat on the focal point and that heat will flow outward to the rest of the metal pretty easily. The only problem I could see happening is heating the focal point up enough to boil it, which might be necessary to heat up a block of certain metals sufficiently via a single point and would make them impractical for use in a focal setup.

And your idea that "platinum can't smelt platinum" is kind of silly. A solar furnace made using silver mirrors could definitely melt silver, for instance, because the array itself doesn't get nearly as hot.

I guess not so much 'can't' as 'shouldn't - it wouldn't be a problem if you were just doing it every once in a while, but if you were smelting ore at a constant rate (as is common for smelters in DF) the ambient heat would get problematic in a hurry.

And since when is HFS that reflective? You're assuming you can just make mirrors out of any substance.

Most metals are relatively easy to polish and extremely reflective when polished, and if you know how to you can buff mud to a mirror shine. (Originating in Japan and called dorodango/泥だんご, polished balls of mud are popular among children and require a lot of time and specific knowledge, but depending on the material can be made almost as reflective as a marble. Polishing dirt and stones to a high shine for no good reason seems like the sort of thing dwarf children would do, but that's neither here nor there.) In any event, I think achieving the temperatures required by HFS forging is ludicrous to begin with, and doing so with mirrors and the sun would probably be impossible even for us.

And the problem with dwarves isn't that they don't have reflective materials. It has to do with the ability and knowledge to build something that huge and that precise with them.

They have a great deal of precision machinery compared to their tech equivalents historically, although I'll grant that given they don't even know how to make lamps it seems dubious that they'd have a grasp of optics as strong as that of gearboxes.

In the end, between how engineering-intensive it is to track the sun even with modern equipment and how slowly temperature tends to move in setups like this even in good conductors, a mirror smelter would probably look kind of like a millstone in reverse: would only require a hauler and the necessary ores, but would operate much more slowly than even a novice furnace operator. Only worth building if you didn't have magma and lack the coal to waste on smelting.

[profit]

(Bangs head on wall)

A platinum mirror array will never be in danger of melting if the focal point is not near it.

HFS materials have better uses than mirrors

The focal point is where the material needs to withstand high temperatures, not the mirrors.

[cparax]

A platinum mirror array will never be in danger of melting if the focal point is not near it.

Platinum is a ductile material which conducts heat very well. Stick it out in the sun all day for weeks and you're probably going to have problems with a machine depending on a bunch of precision-engineered fine mirrors made of it, whether or not it actually melts.

[Footkerchief]

Platinum is a ductile material which conducts heat very well. Stick it out in the sun all day for weeks and you're probably going to have problems with a machine depending on a bunch of precision-engineered fine mirrors made of it, whether or not it actually melts.

Problems like what?  Its melting point is 3214.9 °F and it doesn't oxidize.  Are you envisioning some kind of metal fatigue from sunlight?

[G-Flex]

Less sunlight than normal, in fact, considering it's polished to be as reflective as possible.

[cparax]

Problems like what?  Its melting point is 3214.9 °F and it doesn't oxidize.  Are you envisioning some kind of metal fatigue from sunlight?

I'm envisioning it softening locally and warping. Metal fatigue or any other kind of catastrophic failure would be impossible, but the mirrors would require constant maintenance to be capable of holding a fine collective focus.

Worse still, the minuscule defects introduced by softening wouldn't add up to enough to destroy the setup's ability to liquify metals for some time. You wouldn't notice the problem until it would be a complete hassle to fix.

[Footkerchief]

I'm envisioning it softening locally and warping. Metal fatigue or any other kind of catastrophic failure would be impossible, but the mirrors would require constant maintenance to be capable of holding a fine collective focus.

Worse still, the minuscule defects introduced by softening wouldn't add up to enough to destroy the setup's ability to liquify metals for some time. You wouldn't notice the problem until it would be a complete hassle to fix.

Can you provide a citation for this local softening at barely above room temperature?  I mean come on, it has a higher melting point than titanium and stainless steel.

[cparax]

Surfaces under direct sunlight get substantially hotter than 'barely above room temperature', even fairly non-conductive surfaces like leather or cement. You're thinking of air temperature, which for various reasons tends to rise very slowly and stay within a certain range. Again, I'm not a metallurgist or physicist, but metals that readily conduct heat also readily absorb heat, and air does a poor job of cooling them even when fairly cold. We're talking about temperatures in the hundreds, which isn't as significant in platinum as it would be in meltier metals but which should still be enough to cause small warps and bends which would interfere with a highly reflective and precise mirror.

These platinum mirrors would have to cool down purely by (a) radiative heating into the air and (b) conduction into the armature, and while platinum might shrug off 4-500C temperatures most materials used as an armature for it sure wouldn't. And crack or warp the armature and you've got to build a new one from scratch - you can't just buff it better like you can a mirror.

Again, I think this might be a little pedantic for a game in which you can heat a metal to 15,000+ degrees with magma in a lead crucible. But it's still a good reason to make oven smelting expensive to set up and slow to do.

[alfie275]

Yeah just make it so that light generates heat then the player can build it.

[LegoLord]

Not all the sunlight that hits the platinum is actually going to be absorbed.  In fact, the whole point would be that it would be reflecting the light.

But still.  The mechanisms in DF now and those that will be implemented aren't very precise.  It doesn't take small, intricate gears and axles and springs just to open a floodgate or retract a bridge.  So mirrors are still beyond the tech level of dwarves.

[Derakon]

As for the problems with melting iron: the problem is that iron is a good conductor of heat. You heat up that focal point, and the heat all spreads out into the rest of the metal and then radiates out into the air around it. The big chunk of iron's functioning as a heat sink, just like the one in your computer.

This is completely disregarding the utility of mirrors to melt metal which, I agree, I don't see happening given the tech level we're operating with here.

[Footkerchief]

Surfaces under direct sunlight get substantially hotter than 'barely above room temperature', even fairly non-conductive surfaces like leather or cement. You're thinking of air temperature, which for various reasons tends to rise very slowly and stay within a certain range. Again, I'm not a metallurgist or physicist, but metals that readily conduct heat also readily absorb heat, and air does a poor job of cooling them even when fairly cold. We're talking about temperatures in the hundreds, which isn't as significant in platinum as it would be in meltier metals but which should still be enough to cause small warps and bends which would interfere with a highly reflective and precise mirror.

When was the last time you left a piece of metal in the sunlight and it got "into the hundreds," i.e. hot enough to boil water?  And the average piece of metal will be an order of magnitude more absorbent than even a crappy mirror, so seriously, no mirror is going to get that hot.  And even if you WERE right about that, you also have yet to cite any evidence that any metal will develop heat-related flaws a few thousand degrees from its melting point.

[cparax]

When was the last time you left a piece of metal in the sunlight and it got "into the hundreds," i.e. hot enough to boil water?  And the average piece of metal will be an order of magnitude more absorbent than even a crappy mirror, so seriously, no mirror is going to get that hot.  And even if you WERE right about that, you also have yet to cite any evidence that any metal will develop heat-related flaws a few thousand degrees from its melting point.

The mirror argument is solid, but the statement that severe structural flaws require temperatures close to a metal's melting point is not. Metals decrease in density and expand with the addition of temperature from any prior temperature; the change is extremely minute, but it's independent of distance from the material's melting point unless it happens to pass that point. In point of fact, platinum's (relatively low, but still significant) coefficient of thermal expansion is very close to the (lower) coefficient of glass.

And again, it's not a significant enough effect to kibosh the idea, just to mean that big constructions with mirrors are going to have drawbacks besides the ludicrous costs and highly specific skills and materials needed. Mirrors being used for anything other than simple reflection are extremely fragile and would require constant care.

[Porpoisepower]

At the current stage DF architecture isn't precise enough to build a true parabolic curve even at a largesize... even then It would be more fun to explode invading goblins than cook a fine roast Didn't mythbusters bust the archimedes deathray? I vaguely remember seeing them getting something to work, but the scale wasn't reasonable or something....