Armor Piercing Pennies and Infinite Mirrors

[Kid Orgo]

A while ago, there was a discovery special with devices that used a shaped charge to launch a deformed metal plate at a tank. Could one do this on a small scale with pennies and plastique?

Is the copper in a penny pliable enough to be deformed into some sort of penetrator with a few grams of plasticized PETN?

If so, one could make a wicked grenade out of a few of these in a sphere configuration.

Armor piercing pennies. Imagine the coroner looking at a supposed gunshot victim to find a deformed Abe Lincoln in the back of someone's brainpan.

[a_bab]

I don't think it'll work as a bigger mass is required for the projectile. Your pennies will becoame at best schrapnels, so no more effective than a normal grenade.

[nbk2000]

God damn! You must be psychic or something 'cause I had the same idea an hour ago on the shitter!

I was sitting there thinking about my thread on casting copper platters for use in EFP, and got the idea that you might be able to use pennies to make tiny EFP charges for experiments, since they're copper too, and have seen mention made in a SF training manual on the use of glass watch covers being used as such, so tiny sizes are possible.

The problem with pennies is that they're too thick. A 200mm EFP platter is only about 2mm thick. A penny that's only 20 mm in diameter is 1 mm thick. Scaled up to the size of a 200mm platter, that would make the pennies thickness equal to being almost an inch thick! :eek: Not very deformable. :(

Though anyone who lives near a railroad track has access to a wonderful mechanism that'll squash a penny to the the thickness of a sheet of paper...a train. :)

The penny has to be a pre-'84, otherwise it's not really copper, but rather copper washed zinc. If you can find a section of track embedded in concrete, that's helpful for finding the squashed pennies, since they tend to fly a bit once they're freed from under the trains wheel.

Since you'll inevitably lose some, set a line of a few dozen along the rails, about a foot apart, and held in place with a spot of tape or gum so they don't fall off of the rail from the vibrations of the train.

With the pennies flattened, you could then trim it on a grinder to make it round, insert it into a cardboard holder, and pack a small amount of explosive behind it. But it works better if it's slightly concave, so you might wish to whack it with a ball-peen to dent it on a bit prior to grinding.

Since copper work hardens, and hardened copper shatters, you'll have to anneal it in an oven to return it to a malleable enough state for EFP use.

Using four times the platters weight in explosives is SOP, but given the tiny size of the platter, you'd likely be using 10x just to have enough to stick a detonator in. Highly brisant explosives are required. TNP, DNU, RDX, PETN, NG, or similar would be required. ANNM is too low powered for the job.

Penetration is typically 1/2 of platter diameter for charges over 3", with decreasing ratios for smaller platters, so that should be about (maybe) 3-4mm (1/8") of RHA, at a range of less than a few feet, if everything worked perfect.

Something of such small size, using only a few grams of explosive, might be useful for taking out high-security locks, such as Medeco and KABA, which are normally too tedious to pick/drill. With a "Penny" platter charge, you could blast a hole through the shear line of the locks, allowing you to turn the lock cylinders. Though I'd think an SC more efficient...

Combined with a disintegrating or starch dilantant tamper, you'd hardly make a sound. ;)

[Jhonbus]

Ahh, squashing pennies on the train tracks brings back memories. We found the best place to squash them was just before a station, so the train is moving slowly when it runs over them. Then they don't go flying after they get squashed.

[Mr Cool]

There is a company that sells mini-EFPs (and mini-SCs, too). They're 20mm diameter, and penetration is stated to be no less than 0.5" into mild steel. They contain around 6g of RDX.
That's all "IIRC". The site specialises in EBW dets, they're linked to here, somewhere...
So even small ones can be quite efficient.

[a_bab]

YEs Mr. Cool, you are right. These cute mini-shapped charges are so interesting that I saved the page in my archive so I can provide the link now: here (http://www.risi-usa.com/0products/3sc/page34.html) .

[nbk2000]

I remember the site you mentioned. I also have their catalog. These devices are very precisely made, to a tolerance very unlikely to be duplicated by the amatuer, though they'd work wonderfully for the lock blowing purpose.

[zippoxiv]

For taking out even a numer 5 masterlock a solution of MEKP sensitized with AP not only easy to make but also quite effective. AP crystals are quite tedious to try to funnel into the keyhole of the padlock, but a solution of MEKP and AP can be squirted in with an eye dropper and is fuse sensitive as well. Simply useing a spray insulation foam such as "Great Stuff" you can seal up the lock to prevent the MEKP/AP solution from dripping out. Although quite loud I'm sure its possible to try to muffle the sound somewhat.


Useing a modest amount of MEKP/AP solution you can reduce a no. 5 masterlock to only a few plates and a shackle, the interior placement of the charge works well to break apart the padlock. After I find the time to do some more tests I will post my results as to how much is needed to get the job done. The loudness factor is a bother, and any ideas as to how to muffle the sound would be appreciated, would encaseing it in wet sand possibly do the trick?


Although squirting in a solution of two peroxides is a lot simpler than making a SC, when searching the topic of lock removal I found that the job can be done with just a pipe wrench. A few examples can be found here (http://www.insight-security.com/pf11-1.htm) . The concept of an extremely small SC has its applications, but when all you want to do is defeat a padlock, there are much simpler methods to accomplish this type of task. Sorry if its a bit off topic, just seemed like a better alternative than useing a SC to take out a padlock.

[Kid Orgo]

Heh. Liquid Lockpick. What about the fightclub method of liquid nitrogen or something and a tap from a hammer?

The pre-made shaped charges would presumably (if made from stable explosives) be more convienient for the well-prepared criminal. No one dies if you drop the bottle. However, I really dig the liquid solution idea.

[nbk2000]

MEK is a pretty strong solvent, so I'd imagine MEKP to be also, so I wouldn't use the spray foam. Also, once used, the foam can becomes useless in a few hours. Better would be plain ol' clay.

Please notice that I never mentioned padlocks. Padlocks, like the master lock, often have much more interior space in their bodies, allowing explosive to be inserted. Disc locks, especially, have a lot of interior space, hence the recommendation in my PDF to fill them with AP and blow them open. :)

Medeco's and such have very tight tolerances and very little free space in them, so it'd be more likely to remain intact. Though if you mixed MEKP and NG, that'd be quite a powerful liquid explosive, and likely fuse sensitive. And, given how it's a liquid, you could allow it to seep into literally every little crevice in the lock prior to blasting it.

Do you coat your fuse to prevent the MEKP from soaking in and ruining it?

Noise is an issue but that could be offset by the speed gained by blasting the lock open in a few seconds, instead of drilling/sawing/torching the lock open. A few grams wouldn't be any louder than a gunshot.

LN? How are you going to get it, transport it, and use it? It's a righteous pain in the ass to get, evaporates very quickly, and is dangerous to handle. Also, the lock may not shatter, since the LN method depends on getting the lock body below the temp needed for the metal to become brittle. Some metals actually become stronger when cooled. :(

[Kid Orgo]

Okay, what about engineering some kind of disposable thermal lance? You'd only need a few seconds of maximum heat to burn out a lock. A single unit, with a reaction producing oxygen, and the usual, as in your thread about that o2/steel tubing device. It could be very handy on a breakin. And pretty quiet, compared to a SC.

[nbk2000]

Those are called "Firesticks".

Titanium-Boron-Teflon hand-held rods that produce a heat flux of 10,000,000 watts/square cm at 7,200 degrees F. used for rapid rebar removal as well as melting of locks, hinges, burglar bars, etc. As fast, or faster than, explosive methods with no support equipment needed.

And I've never been able to find a supplier of them either. :(

Though it certainly sounds doable, especially since there's so much patent literature on teflon oxidized incendiary compositions. :)

A rather esoteric idea, but possibly useable during daylight hours, is a frensel (SP?) lens to melt the lock.

Even more unusual are "infinite" mirrors. This are arrays of small mirrors that concentrate the light from their large surface down to a small square of light. Imagine concentrating a square yard of bright sunlight down into an inch. Cheap, quiet, disposable. :D

Best of all? These things act like lenses, allowing for variable focal lengths...10 feet...50 feet...screw the lock, melt the door...through windows...OH, arson! :D

Be parked across the street and set a room on fire by focusing sunlight from a folding mirror array in the back of a pickup truck onto the curtains/furniture/papers.

Proceed to ignite every room on that side of the building on fire in like fashion. Whole building burns down in a matter of minutes from dozens of ignition sites. Let the fire investigators puzzle that one out. :p

How about using said mirror array as a political tool by scorching anti-government graffiti on the sides of government buildings? Computer driven servos allow for set up and evacuation prior to activation. And imagine the difficulty in trying to remove scorched concrete from 10 stories up. :D

Would be a right shame if some office building full of servants of a tyrannical occupational government were to be "accidently" torched in such a manner. Imagine all the buildings that would have to have their widows blocked up to prevent such a thing from happening again.

Poor office workers would go insane from being locked up in a windowless prison not being able to see light of day...kinda like those poor schmucks doing life in underground prisons for speaking out against said tyrannical government...fair is fair after all.

[THErAPIST]

After looking at this topic SWIM decided to experiment with small SC's made with AP. At the moment SWIM didn't have any copper sheeting or any pennies around as the jar full of change was taken to the bank a few days ago to have it all turned into green, so SWIM just used AL foil to hold the AP in place. The first small SC was placed on a piece of 2 x 12 and then leaned up against the mailbox. When the SC was initiated the metal plating used on the back was thrown 176 feet and 9 inches down the road. Yes the distance was actually measured. The second SC that was made was put on a 1x10 which was broken in half and put pieces of wood 3 inches into the ground which the SC was suspended 4 inches above. A metal 3 gallon can was placed up-side-down ontop of the 1x10 and a piece of wood was placed ontop of the can. When the SC was initiated the metal backing was thrown through the top of the can causing noticable damage to the wood that was ontop of it. The SC's only used .5g of AP, so I imagine a couple grams could make a pretty effective EFP if solid backing is used and if it were used at close range (with flattened pennies as the EFP).


Pics (http://i-was-bored.8m.com/mini SC) of the crappy little SC's that were made to try to prove a point...

[nbk2000]

The link you provided doesn't work. Try posting one that works so we can see what you're talking about.

[Kid Orgo]

I humbly suggest that this thread be moved to the appropriate non-WC location. This is getting interesting.


A 8.5x11 fresnel lens is enough to cook a hotdog over a few minutes. I can't imagine it would be cheap to get one large enough to actually melt steel. But I am definitely enjoying the idea of optical weapons. That infinite mirror concept sounds downright wicked. Aim that beam carefully enough, and the investigators might not be able to tell it wasn't a wiring fault that burnt down some fuckers house. Or maybe they would. But if it hasn't been done before, it'd have to be intensely bright and lucky investigator to figure out how it was done.

[knowledgehungry]

The problem with that is that this is more than 2 different topics so opening new threads instead of moving this might be advisable.

[Kid Orgo]

Well, i've started a new threat on the liquid lockpick ideas (giving credit where credit is due)

I'm kinda sheepish about starting new threads, but i'll try to get the rest of this stuff sorted out, if it's not getting moved anywhere.


-edit: Re-reading the thread, there's some pretty good thoughts on small shaped charges in here. With a bit of pruning, I really do think it should be put in a better venue.

[nbk2000]

We now have the ability to split threads into multiple parts. :)

Though this thread doesn't warrant that, otherwise we'd have done it already, eh? ;)

[Kid Orgo]

Well, the MEKP/NG thread generated useful discussion, so I assume I won't be reamed out for starting it.

And I think that might be the only part of this one that would do better anywhere else.

So, um... good.

[THErAPIST]

ARGH! The link worked for me but I know what was wrong with it so Here (http://i-was-bored.8m.com/miniSC) is the fixed link.

[yt2095]

Still on topic, with a little mutagen added.

Ap and Pennies :)

when i 1`st made AP i was more than a little timid regarding it (i`ve since quit making it as i don`t quite trust it enough)
being new and timid towards it, i thought i`de try a few "bench mark" tests 1`st.
i lay a line of AP (about a pencil leed width) on my worktop, it was about 6 inch long, and then lit the end with a wooden spill.
woosh, nice yellow flash ( i use bicarb to wash it) and considerable heat with no residue! i was impressed. and so i did the same again but made a longer line and lay 2p peices along it every 3 inch then lit it. it tossed the coins into the air about 4 inches (i couldn`t stop laughing) :)
next i did a 3 foot track down my hall way and lay masking tape over it then put my coins along it.
CRACK! and the coins went everywhere in a microsecond. shaken but impressed, i got to thinking wouldn`t it be a great excuse to make it at school, if you claimed that you were using it as part of your maths homework in statistical probabilty of the old Heads or Tails crap.
lay the coins heads / tails alternately and set it off then add them up :)

(i know, i should get out more!) :)

[nbk2000]

Well, having found a semi-decent hobby shop right down the street, I picked up a dozen 1" mirrors for under a buck and made a small "infinite" mirror array.

Using poster tack putty to hold them to the back of a hand-held mirror, I made a 4x4 array with a focus at 2'. I was able to melt a pen with it. :)

By twiddling the mirrors a bit, I was able to focus them into a spot at 30' that made a can warm to the touch. The spot was about 3-4" around at that distance, looking like a very bright projection of the sun.

Imagine this...an umbrella whose insides are covered in mirrors that, when opened, presents a very large mirror array with a focal length of a few dozen yards. It may not be intense enough to set anything on fire, but it'll instantly fry eyeballs, cameras, or exposed skin. :D

[Bander]

Although I'm sure most forumites are already well aware of Bill Beaty's page on Infinite Mirrors, providing a direct link for the unaware could not hurt. INFINITELY LARGE SOLAR FURNACE (http://www.amasci.com/amateur/mirror.html). Not only does it cover the topic fully, but on another page he expounds upon the idea with hilarious results.Kindergarten Solar-powered Death Squad: Take a large crowd of children out into the sunshine and give each one a 20cm square mirror. Show them how to aim all of their little spots of sunlight at the same distant object, then stand back and see what they do. Better yet, run away.

[stickfigure]

The only problem I can see with using pennies is that they aren't solid copper they have a zinc waffer in the middle which might cause the penny to fracture into shrapnel instead of deform into a penetrator. To check this out dump a penny into a bath of 20% Nitric Acid and wait about a minute. Also don't breath the gas as it is chlorine and toxic as we all know. Your result should be a zinc waffer with Lincolns head on it.

[nbk2000]

Pennies made in '83 or earlier are mainly copper. It wasn't until '84 that they started making them out of copper washed zinc.

[Dunkelmann]

you could hide the mirror under a screen that allows only the infrared part of the sunlight to get through, this would prevent you from being spotted because of the bright focus.
(or use mirrors coated with IR-Filter)
Another idea is to use IR-Lasers to ignite something from remote, you can use one very stong laser or an array of smaller ones that would be focused. Works even in the dark.
you might even burn somebodys eye from a few hundred feet distance. To aim in the darkness, use a small camera that is IR sensitive, they are pretty cheap.

[nbk2000]

There are mirrors intended for lasers that are tuned for maximum reflectivity in the IR range, but they still reflect normal light, so you'd still be spottable. Plus, they're obscenely expensive. :(

But, there's no reason why you couldn't use IR filter film placed in between the path of the mirror and target to filter out the visible part. If it was segmented so that each mirror segment is individually filtered, then there'd be no heat buildup in the filter material like there would be trying to filter a square meter of concentrated light.

Problem with filtering is the energy loss caused by the filter. They typically cause 80% loss in transmitted energy. :( So you'd need a mirror array 5 times larger than the visible version for similiar power output.

Using IR lasers is ridiculous. Anything powerful enough to be useable as a weapon is going to be incredibly expensive, plus require a generator to power it. A mirror array is cheap, cheap, cheap by comparison.

A square yard of 1" mirrors would cost less than $100 to make. Full sunlight is about 1365 watts of energy per square meter at sea level. Assuming only 1/10th of that was IR, that'd be 136 watts of IR energy for $100. A 3 watt commercial IR laser runs about $40,000. :eek:

Even though it wouldn't have the focus of a laser, I'd rather use the mirrors, thank you very much. :p

[kinetic]

The shaped charges that fire a blunt-nosed projectile you are refering to are called platter charges. The first demolition school I went to we used 1/2" thick milled, round steel plates that weighed 3 pounds each. We cut the bottom out of coffee cans and used the sides of the coffee cans as the sides of the platter charge with the steel plate or "platter" forming the bottom. i packed 3 pounds of C-4 on the back side of the plate and primed with a uli knot of detcord. I made a primitive aming device made of scap tin and then aimed the charge at a compact car. The other students aimed their platters at the same car (a total of 5 I think). All the platter charges were about 7 or 8 meters from the car. The entrance holes were about 6 inches in diameter. They entered the left front door. The exit holes were about 1 to 1.5 feet in diameter and the inside of the car was completely shredded. There was foam from the fron seats hanging out of the exit holes! :D I have photos that I will upload as soon as I get my scanner working. BTW, I have been to several demolitions schools and taught demolitions in a special operations unit so I have quite a bit of the "hoarded" info that mega was speaking of. Lots ao good pics too. I'm not very good with computers though so you will have to be patient with the pics.
Here is an exerpt from one of my manuals on platter charges:

"This device uses the Miznay-Shardin effect. It turns metal plate into powerful, blunt-nosed projectiles. Round platters are preffered, but other shapes will work. The platter should weigh two to six pounds. Steel is the material of choice, though any non-cast metal will work. The explosive weight should be equal to the platter wieght. Prime this chrge from the rear center..."

Also, a slightly convex shaped platter is optimum. Even though the manual says that explosive weight should equal platter wieght, if you were using ANFO, I would beef it up quite a bit (by a factor of 80% since the RE factor of ANFO is .54 and the RE factor for C-4 is 1.34). These have outstanding armor pertration- even more than you would expect.

[yt2095]

thinking about mirror and focal length, perhaps reflective film (like the sort used in hypothermia blankets) stretched over a frame with a small disk and loop glued to the center of the film.
the loop atatched to string and pulled would adjust the focal length from infinate down to a few yards (at a guess).
the frame could be made collapsable for portability, maybe something along the lines of a hang glider slotted tube design?
I`m fairly sure the Hubble telescope uses a similar principal for focus fine tuning using servos.

I wonder what size you`de need for an environmentaly friendly cigarette lighter? :D

[nbk2000]

Rat-shack used to sell a solar cigarrette lighter that was about the size of your palm. :rolleyes:

I dont' think you could use reflective film as is, since it'd need to have a very smoothly graduating curve to it to be able to reflect the light into a coherent spot far away. Any errors in the curvature would be magnified over distance, resulting in a useless (for pyro) glare.

Now, if you could get a self-supporting plastic inflatable reflector constructed, then that might be feasible. It would be compact when deflated, and rigid when inflated, being in the shape of a parabolic dish with one side coated with mylar film to act as a giant reflector. Though it'd have to used in a no wind enviroment to prevent distortion.

[anthracis]

I've done a lot of experimenting with small shaped-charges of AP and HMTD inside pencil lids. The amounts of primaries were quite low, you can imagine, but I pressed them with the tip of the pencil (VERY careful, of course!!) so a cone resulted, inside the mass of primary explosive. A simple fuse was attached before pressing the primary. Sometimes, before using these small charges I put some aluminium foil inside the cone, but I don't think it really improved the results.
The results were amazing: these small shaped-charges made holes of 0.5-0.8 cm diameter in thick (about 1mm) steel coins ("100 lei" Romanian coins). I've clearly noticed that HMTD charges were stronger - it was a lot easier to press, so this may be a cause...Anyway, I know this is not a safe device and it has to be handled with great care.
Then, as I remember there is an incendiary mixture made of plaster of Paris (anhidrous CaSO4) and Al powder. I presume it is easier to ignite than thermite and perhaps it can be used for melting some locks or whatever. One can design the shape of this incendiary mixture in order to get the best results...What do you think?

[Ansgar]

I was thinking along the same lines as yt2095 though just using an ordinary glass mirror as I think they distort the light much less. Problem is just that they can be deformed only very little.

Example... Suppose a circular glass mirror 2mm thick and 1000 mm in diameter could stand having the center displaced 4 mm (I think it is possible) and that the deformation is equal and symetric in curvature. Then that would result in a radius of about 31 m and a focal length of 15,5 m. However at that distance the focalspot would probably never get smaller than perhaps about 3 inches ?

The displacement could be achieved by glueing (?) a bolt on the back of the mirror and tightening the nut accordingly in the back of the frame that holds the mirror. A M6 with a threading of 1 mm per rotation should allow adequate precision in adjustment.

[vulture]

Get yourself a cheapo/used telezoomlens, somewhere in the 70-300mm range.
Point the lens surface to the mirror and use the variable focus and focal length to adjust the distance/concentration of your hotspot.

Much easier than trying to focus a large surface mirror.

In fact, photographers have to keep their lenses covered as long as possible when shooting directly at the sun, because otherwise they'll fry through the shutter mechanism in a matter of a few minutes.

[yt2095]

Vulture,

quite true, I`ve built a device using a similar principal, not solar though. I use a 12.5W halogen 6V bulb and the cariage and battery from a camcorder light attatchement.
mounted there the frosted glass diffuser cover is a lens ripped from an old photocopier.
its focal length is roughly 3Cms to a 2mm spot.
it will set fire to paper through a sheet of glass. this is lame I know, but I`ve even wrote my name on a black tape casette through the clear plastic casing :)

the whole device weighs less than 800 grams and will fit easily into a pocket, I`ve also lit a cigarette for my wife through the car window :)
I`m sure someone will come up with some sort of dark application for this. so I`ll say no more :)

[Lead Storm]

Ive Actuallty done this today instead of a penny i used a Quarter. I inserted the quarter into a bottlecap full of AP and put in a fuse. As a target i used a cast iron pan infront of a tree and the quarter cut cleen through it. The distance between the pan and the bottlecap platter charge was about 2 feet. Ill put up pics soon.

[matjaz]

I'd like to remind that the Sun is not a point-source of light. No matter how perfect a mirror one makes, the diameter of the hot spot at the focus will be around 1/110 of the focal distance, since the Sun's apparent size in the sky is half a degree. It's an inherent limitation, so additional lens, whatever... won't help here.

This means two things:
1. The bad thing is the few kilowatts that you can collect with a reasonable mirror array will be spread over a square foot if you are 30m away from the target. Not much of a heat...
2. The good thing is that if you make a segmented mirror, made of many smaller ones, you don't need to have really small ones and therefore that many of them. They can be about as big as the hot spot, without a bad loss in heat concentration.

Oh, a variant on the kindergarten death squad. Imagine a football game. The entire spectator's tribune (that how you'd call it?) well visited by our forumites. On a sunny day. With a 15" mirror each. You get the idea. It adds to around one megawatt. (Anyone been playing with a lens and ants as a kid?)

[matjaz]

Lead Storm,
how thick was your target dish? And, how did the coin hit it? Do you have a round hole or a narrow one, indicating the coin cut it with its side? I wonder for how long can one make the coin fly more or less flat-on...

[nbk2000]

There's a trade-off made when choosing mirrors.

Many small mirrors increases effective heat per square measure of mirror surface, though with decreased range.

Larger mirrors increase distance, but with decreasing effective heat as the size of each mirror cell is increased.

Please read the link provided by Bander before commenting on it.

[matjaz]

NBK,
this William Beaty chap got it wrong so let's not wave his web page as the reference work.

Below, I tried to draw what really happens. The shape of the hotspot is a combination of a circle (of about 1/100 of the focal distance in diameter) and the shape of a mirror segment. (Or, as mathematicians would call it, a convolution of the two twodimensional distributions.) The size of the circle does not depend on the size of the rectangle!

You can reduce the spot size - and make it hotter - by either reducing the focal distance or mirror segment size or both. The guy blew it because the blurring effect is just more _visible_ with smaller mirror chips, as the hotspot becomes evidently nonrectangular (you get to convolve a smaller rectangle with the same circle). But the "amount of blurring" is the same - he didn't figure out that one. Every point on a flat reflective surface has the same "pinhole effect".

It also makes it obvious when you consider the limiting case where the mirror chips are reduced to very tiny ones. That would be a smooth concave mirror and it is of course ridiculous to claim that the useful range of it would be zero.

Once again, for a fixed total area of the furnace, you don't lose anything in performance or range of the system if you use smaller segments. So there's no trade-off here. You indeed gain. It's just that you don't gain much if you intend focusing it far away. So it's not worth the extra work. In this spirit, you can still use Beaty's numbers, since he got those right.

[Damn, no attachment button in this thread...? And I've produced such a beautiful illustration... :( Oh well, if you need to have a look at it, I can email it to you.]

If you click the "new reply" button instead of using the quick reply, you should be able to attach your image

I did that, but in this thread, the "Browse" field just doesn't appear. I do get it in other threads but not in the ones of the Water Cooler. I guess that's to prevent smartass newbies like me from overloading the forum. :)

[NightStalker]

Having built a small one, I'd say there's definitely a loss of heating effect using large pieces of mirrors, compared to many smaller ones, for the same area.

And the smaller pieces don't reach as far as the large pieces, at least as far as heating is concerned, so I'd have to disagree with you there too.

Have you built one yourself?

[matjaz]

NightStalker,
no, I haven't built a furnace like that.

Yes, we absolutely agree on your first point, I'd say that's the trivial one. As Beaty notes correctly, "more beams on the same spot".

As for the second one, I will take a guess at what went wrong with your trying smaller segments. It must be quite a pain to focus them well because the light patches are smaller and need to be aligned much better to get an overlap. But there is no inherent increase of spread from each segment just because the chips are smaller.

(Another point: with smaller mirrors on the same board, you might have had the same gap width between mirrors. This effectively decreases the total area.)

[edit by matjaz: rephrased]

[edit2 by matjaz: No, even the alignment problem couldn't have been the cause. Smallish pieces can be off-focus as much as the bigger ones, still giving the exact same heat on the hotspot, since there are many of them to average off. The alignment issue only comes in to allow even some extra gain with smaller ones if you're really careful. Another option is loss on the chip edges if they are not cut cleanly or if they are thick and tilted a lot with respect to the mainboard plane (short focal distance). Other than that, no other mechanisms I can think of....]

[Jacks Complete]

I still have a box full of 2x2cm mirror squares I cut up years ago for this. I could never get the damned things to stay still, and so gave up. (It would have been wildly heavy, too.)

How are people keeping the mirrors solidly in place?