This was a project I did in a group for my Materials Science class. Since I had access to a laboratory full of materials testing equipment, I'll give you some extra scientific data. But if you happen to have access to casting equipment at home, you should be able to produce similar effects!

Necessary materials:
-something to smash glass
-materials to make an investment mold (wax, investment plaster, flasks, a vacuum sealer)
-something big enough to hold glass over investment at around 850 degrees C
(Please read through the steps first- this materials list is probably not comprehensive.)

I'm not an expert- none of us had tried working with glass before this project. If you have advice, please comment!

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Step 1: Make wax pieces

Make positives of whatever it is you plan to cast. Sculpting, carving, or braiding thin wax pieces is easiest.

Another option is to copy an existing object using silicone molding and a wax injector. I used mold-making rubber, but here is an instructable for a different way to make silicone molds. Make sure there is an opening from the side of your mold to your object so that you can inject wax with the wax injector. I made several wax copies of a phoenix figurine using the silicone mold pictured.

The objects which cast best in glass for us were the solid figurines, such as the animals pictured. However, some of the more delicate objects (some rings, a phoenix) also came out well.

Attach your objects to a wax sprue.

Step 2: Make an investment

Picture of Make an investment
Place your sprue into a stainless steel flask and seal the holes. You will pour your investment into the flask to completely cover the wax pieces.

Follow the instructions for your investment mix to properly mix and set your investment.

Put the investment in the furnace to melt out the wax, leaving holes for your glass to fill.
trufrontier4 months ago
Bore us? I thought it was fascinating. I've just started working with glass. I'm using low melt 90 & 96 coefficient frit (very finely ground glass).

I take it that the word "investment" means whatever mold your actually sacrificing in the kiln because one needs to destroy it to get to the final molded glass object.

Anyhow, the frit I'm using melts at about 2000 degrees and I leave it in the kiln for 2 hrs. I've had my glass come out opaque like yours and also nice and transparent. I thought mine came out opaque because I fired it either too long, or too hot, or both.

I enjoyed your Instructable and learned something, too.
SelkeyMoonbeam (author)  trufrontier4 months ago
Thanks for the great comment! The frit casting sounds significantly more effective than my version; I wish I'd asked around online more before the project, as that would have been a good process for us.
I also wonder if the smashed glass we used became contaminated during the smashing step (grit from the pavement), which could also have contributed to its oddness as well.
I'm actually molding a cement lizard for my front yard. I wanted the eyes to be shaped like quarter moons and tried to cut tile and shape them that way, way too hard. so I stumbled onto molding glass while I was on Amazon and thought I'd give it a try. I'm flying by the seat of my pants as I'm using a kiln I built myself out of bricks and a propane tank, Raku style. Everything's trial and error with me, but I noticed that when I backed off with the heat and shortened the time my glass started to clear up.

I found a lot of info reading the comment sections under products on Amazon and, also, watching You Tube on the subject.
Cooling metal too quickly can generate cracks. But almost always leaves a less-desirable molecular structure.

Don't cool it faster than you have to.
SelkeyMoonbeam (author)  Chris Logan1 year ago
Would you recommend quenching & annealing?
Quenching will cause immediate fracturing in the glass, regardless of type. Controlled regulation of the temperature from the annealing point, through the strain point, and then down to room temperature is required. Rapidly cooling the glass though the strain point will cause a tempering of the glass but this is not done with the casting method as the imperfections in the surface would negate the process. Each glass will have its own temperature and time range requirements and continually increases with thickness of the casting.
Not unless you have very specific requirements for material properties.

Cooling iron, zinc, steel in cold water will leave microfractures throughout the surface if it doesn't shatter the part outright.

Cooling aluminum, magnesium results in a stupidly soft and unmachinable alloy, regardless of your starting material. 6061 after a water quench ends up with properties like 1001.

Just let your parts air cool. You avoid putting unnecessary molecular and physical stress on them.
Air cooling avoids that stress. And results in a part that is easier to machine or handwork finished.
So there are a couple of issues that will go into how you should anneal your glass, glasses behavior through temperature ranges is very different than for metal. To give a brief explanation, all glasses contain specific COE, or coefficient of expansion, which determines the rate of contraction though its strain point on the way down to room temperature from the annealing point. For most modern soda-lime and lead based casting glasses this is between 104 and 85 COE, while boric based pyrex glasses have a common COE of 33, higher he number, the. In general for casting the higher the COE the lower the viscosity of the comparative glass at the same temperature, lead based glasses (104 COE) will begin to melt as low as 400*C while pyrex (33COE) would need to be brought to above 1500*C for any effect. Most Casting glasses have a investment casting temperature of 850*C- 870*C, higher temperatures will further reduce the viscosity, but most molding materials will begin to rapidly deteriorate higher than this.
3DTOPO1 year ago
Thanks for the info. I am quite familiar with metal casting investment, but I am not sure about chemistry compatibility with glass. (I was not able to post under your reply; seems to be a bug with the captchas)
SelkeyMoonbeam (author)  3DTOPO1 year ago
Oh, I see.
The same investment should work; the teacher whose materials I was using had only worked with metal before, and I just used the metal materials for glass.
3DTOPO1 year ago
Thanks for the info. I am quite familiar with metal casting investment, but I am not sure about chemistry compatibility with glass.
3DTOPO1 year ago
What type of investment did you use? Can you share who you purchased it from too? Thanks!
SelkeyMoonbeam (author)  3DTOPO1 year ago
I'm sorry, but I don't actually know... this was done in a classroom setting, and the powder we made our investment from was purchased by the teacher.
However, here is another excellent casting instructable: and the author of that one recommends 'satin cast' by kerr for detailed jewelry casting in the comments.
Additionally, your graphite, that's dissolving away, is just oxidizing into oblivion... CO2 generation.

I'm sure some of it was absorbed by the glass.
First of all, this is really, really cool.

Secondly, what the hell is up with the purple gaffer's glass? Are they nuts?!? Who puts that much lead in something? Obviously, this is not directed at you, selkeymoonbeam, - just whatever criminally insane freak that decided to make that available to the public without a giant - WARNING! THIS GLASS IS INSANELY TOXIC! - label all over it.  Lead in glass will leach out in 5% (vinegar-level) solutions of acetic acid - I don't want to think about how much would leach out in stomach acid if someone (i.e. someone under the age of six) swallowed a bead made of that glass.  <rant ends>

Thirdly, I wonder if it's the potassium that makes the wine bottles green.  European forest glass frequently had a greenish cast to it, and it seems likely that it was because glass makers were using ash/potash for their various alkali carbonates, which would contain (in that region) I high percentage of potassium.  That is just very interesting ....

Fourthly, that is just a very cool instrucable.
My understanding is that the old scandinavian / german glass is green because the soil used in the batch contained iron oxide. Today those impurities are removed, and Iron Oxide and Chromium are added to make deep green glass, like wine bottles.

When glass is re-melted over and over, it tends to develop a greenish tint. This might happen because a piece of metal from the blow pipe was re-melted, or from something in the furnace like the heating elements degrading (my guess).

I wouldn't hesitate to put a piece of yellow glass in my mouth (even though it contains cadmium), but if I'm melting it in the fire, I'll definitely be working under a ventilation system. I don't think using lead crystal glassware on a day to day basis poses any serious health risks, but storing alcohol in it for any long period of time should be avoided.

This instructable is great! I enjoyed the laboratory approach very much.

That was my understanding too, but their chemical analysis showed no iron or chromium in the green glass from the wine bottles. I had assumed it was a form of iron (II) hydroxide that caused the green hue. Maybe the amounts were too low to be measured.

I think that the general feeling has been that lead crystal is fairly innocuous used as intended - i.e. not for storing food. However, as lead can leach out in measurable amounts if even low concentrations of a weak acid - like vinegar - is stored in it overnight, I think that the conventional view on lead glass is about to be revised. If there are small children in the house, I would strongly advise against using lead crystal at all. I would imagine that the same would hold true for other heavy metals in glass, although I haven't seen any studies on the subject, so this is just a guess.

The extremely high levels of lead in the purple glass used here was dismaying to me because, if it were made into a bead, that seems like the exact sort of thing a child would want to put in his or her mouth. Makers of that kind of glass always seem to have this thought-bubble floating over their heads "this is harmless as long as people use it the way I intend." The reality is that the instant this product leaves their posession, its use will probably bear little resemblance to their initial intention, because people only see that the color is pretty and don't understand how the color is achieved, or the dangers the product can pose.
Haha thanks for reading!

The purple glass- I'm not sure what percentage normal lead glass has of lead, but leaded glass is still pretty common. Many people have lead glass decanters for alcohol- which is really bad, because it takes about an hour for your decanter to poison your wine. Also, Swarovski makes a lot of little figurines that people give to little girls, and that is high lead content as well. Some online sources said that lead crystal wineglasses could be all right, though, since you drink it more quickly... but yeah. They should really label that stuff more clearly. For this particular glass, I believe it was professional glassblower's glass, and I got it via my professor, so it might not be "publicly" available per se.

I had speculated that potassium was a coloring agent, but found no evidence to support it. Another interesting note on the wine bottle glass, though: since it was for storing wine, it's opaque to UV light!
These look amazing, great work!

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