I am considering building a small two-bedroom cement-block house with a ferrocement roof. Ferrocement is Portland cement plus rebar and chicken wire. Very flexible until the cement is added. I would like to add simple skylights made of solid glass. Cheaply. So I am mulling over a simple way to cast a skylight from clear beer-bottle glass. I see that beer-bottle glass is held in much disdain in this forum, but hear me out! Originally I was thinking round, but now I think square would be easier to fit into the ferrocement mesh. Probably 6x6 inches. I am imagining something like a glass brick, but square with rounded corners. Perhaps 1" thick. I'm proposing free materials (clear bottles from the trash heaps all over Boca Chica), so might as well make them thick for strength. By the way, I saw the warning about mixing glass with different COEs. So bottles from the same brand of beer. So here is my minimalist idea. * Make a wooden prototype. Piece of sanded wood, 6" by 6", 1" thick, with rounded corners. * Press the wood into leveled sand until the wood and the sand are seamless. * Carefully pull out the wood. Eyebolts in the wood. * Add a measured amount (by weight) of crushed glass to the depression in the sand. * Melt the glass with a blow torch. * Let cool. * Brush away the sand * Optional: frost the surface of the glass for better light dispersion. Emory paper? * Insert the glass skylight into the ferrocement lattice. * Mortar around the glass. I assume the roof would be created on the ground in panels, 4' by 12', and lifted into place. Thoughts? Peter
From someone who has tried to melt glass with a blowtorch, that's not going to come close to softening anything much over a five cent piece in size. You could buy glass blocks for not much over $20USD and they have good thermal insulation properties as well: Molten glass of anything thicker than plate glass usually needs slow cooling in a furnace over days to avoid stress cracks on cooling. If you had a large oxy-acetlyene torch and moved it over some glass, melting small sections it as you go, it would eventually crack as it cools.
Mark, thank you for your response. I will definitely look for glass brick in the DR. If I had to buy it in the US, it costs $5 per pound to have it shipped here. In my youth I used to melt bottles in our coal furnace.
Watching some Youtube videos on making glass or glassworking shows furnaces running at 1300 degrees C and glass being heated for days at a time and then a cooldown of days afterwards to avoid internal stresses (revealed with polarized light and polarized viewing film). Reusing glass is much more efficient than making it from scratch but even so it's going to be a major undertaking to get a fused chunk (maybe in an electric furnace) that will require sawing out if the mould it's fused into and could likely shatter doing so. You can see at the 2:40 minute mark, when making optical glass, the useable glass is in broken chunk form that has to be sawn into blanks for use. Even the glass blowers in the second film have a decent size furnace for working relatively small amounts of glass. Optical glass from raw materials: Glass furnace relining, showing a small glass working furnace (With lots of asbestos gloves):
At the end of the second video, the artist is making something vaguely swan-like. Exposed the way it is, it will cool rapidly. Must it be later annealed?
I once took a broken glass art piece to a glass blower at the local markets for repair, he said he couldn't do it without a furnace to preheat the whole piece as locally melting a spot to fuse it would cause stress cracks elsewhere. I suspect art pieces are full of stresses. In fact the reason potassium glass (gorilla glass) is so tough is because it has none of the internal stresses caused by the sodium atoms that normal soda glass has. Those art piece glasses would have all sorts of oxide additives and might be more leaded glass (flint glass) than soda glass (crown glass). What those two videos show is the serious amount of energy applied over a prolonged time to make glass melt and in one case it results in a whole lot of broken chunks that are then cut up and used. I suspect it's one particular rathole you don't need to go down if all you want are skylights. One alternative I used for my concrete block and corrugated iron roofed workshop was a surplus 200 Watt 36V solar panel that I cut into and reconfigured as 12 Volt and then ran figure 8 wire to 12V strip lights throughout the workshop. This let me light upstairs and downstairs brightly enough to see well and would be difficult with conventional skylights. Also the roof drainage wasn't interrupted by cutting holes.
Interesting. I'm reading your post with my Moto/gorilla-glass phone. (I only buy Moto phones, and I have never broken or scratched a screen.)
So, bright idea! Don't melt the bottles at all. Just mortar them into the ferrocement sideways. Ferrocement is fairly thin, about an inch thick. So the side of the bottle would stick out on the top and bottom of the roof and let the light thru.
>>During its manufacture, the glass is toughened by ion exchange. The material is immersed in a molten alkaline potassium salt at a temperature of approximately 400 °C (750 °F),[38] wherein smaller sodium ions in the glass are replaced by larger potassium ions from the salt bath. The larger ions occupy more volume and thereby create a surface layer of high residual compressive stress, giving the glass surface increased strength, ability to contain flaws,[39] and overall crack-resistance,[40] making it resistant to damage from everyday use.[38]<< So they are not replacing ALL of the sodium with potassium, just on the surface. They create evenly-distributed tension at the surface!
There is also a type of glass made with potassium carbonate, calcium carbonate and silica known as "Hard glass" which is harder, melts at higher temperatures and is resistant to chemical attacks. I'd thought it was on a par with gorilla glass as it had no sodium content but I could be wrong about that.
As someone who has melted beer bottles in a glass furnaces, DON'T DO IT! There is specific glass MADE FOR CASTING. But get out your wallet. I was screwing with Dalles a year or two ago for some bronze projects. The thread is lengthy, but it's got a lot of the leg work done for you already. https://forums.thehomefoundry.org/index.php?threads/chunk-jewel-lamp.622/ Casting metal is 100 times easier than glass. It is a true pain in the ASS!
Why don't you just cut the bottom couple inches off the clear bottles, arrange them in a decorative array bottom side up, secured with mortar, in your ferrocement roof? Best, Kelly
If you want something cool, look up Dalle De Varre. They are mortared or joined with epoxy. Once a panel is built, it's damn near bullet proof. But VERY HEAVY. The dalles are around an inch think. To save money, buy blems.
It's Star/asterisk design. Six beer bottles (clear glass). Replicate around ferrocement roof/ceiling. (I really appreciate all of the feedback. You guys are great!)
Wow! Heavy is ok. The ferrocement is about an inch thick, so these could blend right in. https://en.wikipedia.org/wiki/Dalle_de_verre This might be a future option when my budget allows. Right now I love the idea of picking up free beer bottles blocks from my house! (People here in the DR just throw their trash in vacant lots. :-( )
