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[Mikademus]

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?

As a simple courtesy, would you mind providing temperature figures in a sensible format (a.k.a. Celsius)? Most readers, me included, are lazy and won't bother with looking up the topic or manually do F->C conversion.

Thanks.

[Derakon]

It doesn't really matter once you're in the thousands of degrees; either way it's too damned hot.

Keep in mind that the hottest you get on the surface of the Earth is only around 120°F.

[Footkerchief]

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.

Yes, and?  How would the miniscule, uniform expansion of a platinum sheet cause "small warps and bends"?  Even if the sunlight weren't falling uniformly on the panel, the temperature differences are so low and the thermal conductivity of the platinum so high that thermal expansion will remain uniform for all purposes.  The mirrors are going to warp infinitely more from their own weight than from sunlight.

I tried to do your research for you by looking for papers on the degradation of the surfaces of mirrors in solar furnaces, but everything was discussing degradation from dust accumulation, etc. rather than from sunlight itself.  Your best bet for an argument at this point is researching the light intensity needed for laser ablation of various metals.

As a simple courtesy, would you mind providing temperature figures in a sensible format (a.k.a. Celsius)? Most readers, me included, are lazy and won't bother with looking up the topic or manually do F->C conversion.

Thanks.

Sure, I'll try to remember.

[SirHoneyBadger]

As far as the "Archimedes death-ray" goes (just to play 'Devil's Advocate' for a moment): Were they testing to set the hulls of the boats on fire? Or just the sails/rigging/etc.? It seems to me that certain sails (made from dry reeds? cloth?) might be easier to set on fire than heavy, damp wooden hulls.

And a much easier target, for that matter.

[profit]

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.

The mirrors are used for simple reflection.  The mirrors in real life do not have these problems in solar furnaces.

Please check real life solar furnaces before you post any more on this.

*just to clarify I believe this is NOT something that should be in dwarf fortress but I also know that inside reality solar furnaces work without mirror trouble.     

Please concern yourself with the ability to make a perfectly flat mirror.  Or the ability to make a highly polished reflective one.  Or the ability to track the sun. Or the ability to make a lot of them.   The mirrors being deformed, damaged or otherwise due to temperatures (except extreme cold) is just not a problem in real life.

[Jadael]

The simple answer is, if it's possible to make a flat, reflective mirror, you don't need to bother with making a giant parabolic dish. Just set up a while bunch of tiny mirrors and aim each one (you can do this by just looking at the bright spot of light it's casting) at the same spot.

The "Death Ray" I linked earlier is an example of this. You don't need math, or even planning to build one. Just a shitload of small mirrors. At about half the size of a door, it's able to set toys and things on fire.

The issue is making mirrors. I'm not actually sure how difficult it is to make a small mirror, but I've polished steel by hand and it's a lot of work but not impossible to get it to a mirror shine by hand.

Another issue is how useful this really would be.

[LegoLord]

When you "mirror shine"  do you mean you could see a clear reflection of yourself?

[SirHoneyBadger]

I've shined the face of a copper penny to mirror brightness (yes, I could see a clear reflection of myself) on a piece of porcelain before, so it's definitely not impossible. Several hours of work, but doable.

[G-Flex]

Just because you can see yourself doesn't mean it's a good mirror, though. You'll see yourself in something that reflects only a small percentage of light, as long as the ambient lighting is actually decent and the thing involved is smooth.

[tsen]

Given the obsessive attention to detail, immense patience and skill, and general stubbornness of dwarves, I'd say they can definitely put together a solar oven, with the right materials. Whether it would be any use as a weapon is highly suspect. I could see optic magnifying sights on their crossbows though, that wouldn't be too high tech.

[Porpoisepower]

As far as the "Archimedes death-ray" goes (just to play 'Devil's Advocate' for a moment): Were they testing to set the hulls of the boats on fire? Or just the sails/rigging/etc.? It seems to me that certain sails (made from dry reeds? cloth?) might be easier to set on fire than heavy, damp wooden hulls.

And a much easier target, for that matter.

Please bring on the satonic advocation!

Back to the Deathray
Hull, but they had it nice and tarred up in period fashion... plus Sails do get we and I do not have any data on ignition tempetures of humans and goblins[edit... that might be in the raw's I'll have to check]