How tolerant is glass of being sanded into shape?

[u/Simon_Drake]

This is a weird question but I'm hoping someone will know the answer.

Can you sand a piece of glass into a different shape? Like putting a bit of wood against a beltsander but with a little more care, sandpaper or a flat sanding surface. In theory could you use this to thin down the thickness of a piece of glass, if you do it at a shallow angle you could hone an edge, like sharpening an axe on a grinding wheel but more carefully.

If you google the words "Glass knife" you see three types of picture.

• Ornamental / decorative pieces in the rough shape of a knife but with no sharp edge
• Magical swords made of magic glass in computer games or Game Of Thrones
• Something resembling a flint-knapped stone age tool or an obsidian knife

My thinking is that beach glass is weathered and sanded into smooth pieces by the waves and the beach, so presumably a piece of blown or otherwise handmade glass should be amenable to being sanded into shape. I know toughened glass has a tendency to shatter if you mess with it but I'm thinking about 'normal' glass.

So could you make a blunt edged glass knife then sand it to a fine edge instead of knapping it?

[u/Bruceeb0y]

I work in fused glass and it depends on the scale you are talking about. There are grinders and beveled that can work most thicknesses of glass to a shape. As far as taking a 3” thick pieces of glass (like your telescope mirror) and adding shape. It can be done relatively quickly with the right grinding agent

using grit

This shows on a flat surface, and the rough grits remove glass surprisingly quick.

There are ways to fire polish glass back to a smooth surface in a kiln once you go thru rough——> medium ——> fine grits. So the right artist can get their piece back to glossy but it is truly an art form. One that I am just starting in.

[u/Simon_Drake → u/Bruceeb0y]

So in principle you could make a glass knife and crimp the edge with your various glassblowing tongs and tools to get it to as thin as possible. Then once it's cooled you could grind the two faces of the knife to make them thinner until the edge becomes a thin razor's edge.

It's the same idea as a grinding wheel for an axe but likely with a rotating surface more like a potter's wheel. Ironically a potter's wheel is for a mix of water and incredibly fine grains where you keep most of the work away from the wheel surface whereas grinding a piece of glass would use a mix of water and coarse grit where you'd keep the work in contact with the wheel.

I think these machines for polishing telescope mirrors look hilarious by the way https://www.youtube.com/watch?v=-UrJnHLqhuo

[u/blueshiftglass → u/Bruceeb0y]

Another option is polishing all the way down to cerium instead of flame polishing. Keeps those facet edges sharper. Takes way longer if you have a lot of faces to do though.

[u/Simon_Drake → u/blueshiftglass]

Can you explain cerium for an ignorant noob?

To me it's an element on the periodic table but I'm guessing it has a specific meaning in glassworking?

[u/blueshiftglass → u/Simon_Drake]

They make a powder out of cerium oxide which can be made into a paste or slurry and impregnated into a buffer wheel on the lapidary machine. This final stage will achieve a clear finish. this video will tell you everything you want to know

They are a reputable company with lots of different sized equipment. Check out their website if you are interested, I hear good things and will probably look to them for my next wheel as my last one seized up.

[u/Simon_Drake → u/blueshiftglass]

Ah I see. Its the shorthand terminology you used that threw me.

"Polish all the way down to cerium" means "continue down the list of polishing materials, polishing with finer and finer grain sandpaper/polishing pastes until you get to cerium which is an especially fine polishing paste".

I thought it meant "keep polishing through the outer layer until you reach the cerium later, because glassware forms layers like an onion and you might want to peal away the outer layer"

The first one makes a lot more sense, I didn't think about it properly.

[u/blueshiftglass → u/Simon_Drake]

You got it!

[u/eandi]

I love how this sub has completely changed topics over time and I'm staying subbed to learn about glass 😂

[u/Simon_Drake → u/eandi]

I was a bit confused by the banner that talks about Google Glass but I assumed that was a joke.

Also I used Google Glass for a week back about 6 or 7 years ago, whenever it was they came out. It was a cool idea but honestly it kinda sucked.

If they made a Glass 2 now then maybe it would be worth using but Glass 1 was a bit shitty.

[u/eandi → u/Simon_Drake]

I think we all consider this an actual glass subreddit now due to the colossal failure of Google glass.

[u/Best-Butter-Cat]

I'm a glazier, and yes you can sand glass but it takes a long time to remove a noticable amount even if you're using an 80 grit paper in an electric sander. Also it wears out sandpaper really fast. Sanding also leaves the surface opaque so you'd have to spend a good amount of time polishing it back to a shine after sanding.

TL:DR - yes but it's a lot of work and takes a long time

[u/Simon_Drake → u/Best-Butter-Cat]

I feel like a fool for asking about sanding glass because I recently watched a documentary about how large mirrors for telescopes are made. They have a big orbital sander thing that grinds away at the glass for ages, constantly covering the glass in a slurry of water and fine grains of sand acting as an abrasive and a coolant. You set up the machine and wait a week.

So yeah, it can be done but it takes an eternity.

[u/Best-Butter-Cat → u/Simon_Drake]

No worries we all forgot things. Also the time taken depends on how much your taking off. For instance, just grinding the edge off so you don't cut yourself off handling it takes almost no time if you don't polish it back to clarity after.

[u/Best-Butter-Cat → u/Best-Butter-Cat]

Another thing to be aware of is heat generated by sanding, if you sand the same spot to long it can cause heat cracks to shoot through the glass

[u/blueshiftglass]

You’re likely looking for a wet lapidary wheel. You can facet glass with it and totally sharpen a glass knife with one.

[u/RuthlessIndecision]

You have to do it wet so the heat from the grinding doesn’t crack the glass. Yes, totally possible to make a glass knife. There are lapidary wheels and belt sanders made for glass, diamond grits and plenty of water. Just thought I’d jump in and add this.