crystal vs. glass

[mikan]

What's the difference (apart from price) between crystal and ordinary glass? And what's the big deal about "cut" crystal, anyway?

[rjk]

To be called 'crystal', glass needs a certain lead content, in the form of lead oxide. The lead gives it a higher index of refraction, so it sparkles more.

Glass items can be molded (look for lines down opposite sides of a cheap piece, where the halves of the mold didn't line up), which saves money but gives the design a soft rounded finish. Cutting with an abrasive wheel takes a lot of highly-skilled labour, but gives the design sharp, clear edges that catch the light better.

It's easy to see the difference if you set two pieces side-by-side.


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Bob the Random Expert
"If we don't have the answer, we'll make one up."

[mikan]

Thanks, Bob, for your Random Expertise!
When looking at crystal not long ago, I also felt that it was quite heavy. Is it heavier than regular/ordinary glass?

[Ringo]

It's easy to hear the difference, as well. Crystal rings if you tap it with, oh, a butter knife.

[Ringo]

My last post caused me to do a little research in the kitchen. Glass will ring (more like ding), but the note you elicit from glass fades much more quickly than that from crystal.

[Undead Dude]

Lead crystal is also less brittle (more flexible) than regular glass. This flexibility gives it a greater ability to resonate, which is why crystal is sometimes used to make musical tones, an why it can be shattered by a voice under the right circumstances.

When I was a teenager, my mom had a bunch of crystal, and from time to time I would drop them on the floor (on accident). I was always impressed by two things that were both a result of this flexibility-- they almost never broke, and when they hit the floor, they made a very loud tonal sound.

[Gaudere]

And with crystal, you can dip your finger in water and run it along the rim and make it "sing".

[mikan]

Ah, yes - I forgot about whole singing wine glass trick. That alone makes crystal worth the extra price.


But back to the "cut" crystal: I guess what I was wondering is whether or not it's possible to cut ordinary glass in the same way as crystal? Does the brittleness or ordinary glass make it less amenable to being cut, or is there simply no point to cutting it in the first place (because, after all, it's just plain ol' glass)?

[mikan]

Make that "brittleness OF ordinary glass." (Grrr...)

[Moonshine]

The heaviness of crystal is directly attributable to its lead content, the more lead that is put into it the heavier it becomes and the better it refracts the light. There are two types of cut used in making crystal ornaments; I forget their names but one is the actual "cutting" which produced ridges and channels, the other is abrasive which matts the surface down. Both of these take a very long time to learn how to do properly, so my guess would be that glass ornamentation is moulded and crystal is cut, simply because of the cost of labour.

Another thing I learnt on my tour of the Waterford factory is that those cut crystal vases look really really nice under the powerful spotlighting in the showroom, but not nearly as splendind in the more normal lighting of your house.

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It only hurts when I laugh.

[Omniscient]

Correct me if I'm wrong but the fundamental difference between crystal and glass is that crystal has a crystaline structure, glass is an amorphous solid. Lead oxide aside, virtually all the advantages of crystal are due to its crystaline structure. The *singing* is caused by resonation, glass doesn't resonate under typical circumstances. Crystal has a better clarity and more strength due to the structure.

[David B]

I don't think "crystal" actually does have a crystalline structure. Yes, that means "crystal" is a misnomer. It's still glass, just a specialized type.

[Omniscient]

Quartz is crystaline Silicon, correct. If were concerned with Quartz crystal then it obviously is, I assumed that lead crystal was a leaded quartz, if not I'm curious what about adding lead oxide to glass improves the physical qualities of it. Is it still an amorphous solid?

[Undead Dude]

Quartz in crystalline silicon dioxide. I am almost positive that "lead crystal" is amorphous, primarily for one reason. It doesn't cleave.

EB classifies lead crystal as a subtype of glass ("lead-alkali-silicate glasses"), and refers to glass in general as an amorphous solid.

Crystal can use other metals besides lead. Silver is used by an Austrian company, Swarovski. The cost is generally too high for stemware, but they use it for some spectacular collectibles.

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Sue from El Paso
members.aol.com/majormd/index.html (http://members.aol.com/majormd/index.html)

[Omniscient]

So what is crystaline Silicon, Si4? It must be quite similar to diamond, and thereofre very useful and valuable.

[Undead Dude]

Going back to EB again:

Pure silicon is a hard, dark gray solid with a metallic lustre and with
a crystalline structure the same as that of the diamond form of carbon, to which silicon shows many chemical and physical similarities.

It doesn't seem to say what the crystalline form is used for. Of well. :)

[rjk]

Omni and Undead, I'm surprised! Crystalline silicon is used for the chips you computer is running on!

Pure silicon (I mean really, really pure; they start with 'reagent grade' material, like a chemist would use for research, and refine it from there) is a pretty good electrical insulator, but if it's doped with tiny amounts of other elements, it can conduct electricity in interesting ways.

But cut crystal is prettier. :)


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Bob the Random Expert
"If we don't have the answer, we'll make one up."

[rjk]

"the chips your computer is running on"


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Bob the Random Expert
"If we don't have the answer, we'll make one up."

[aseymayo]

And just to confuse you more, Mikan, "crystal" can also refer to the color of the glass (or lack thereof). If you start collecting Depression glass, you'll find many patterns come in pink, green, amber, etc. and "crystal." This doesn't mean the glass is higher quality, but that it is "crystal clear."

[mikan]

Yikes. I think I'm going to stay away from antique stores until I forget about that last tidbit.

[Doobieous]

I've seen a pure sample of silicon that was used when they create microchips (it looked kind of like a top). It was dark gray and silvery in color. Quite light for its size. When I saw it at the front of the class on one of the desks I thought it was a chunk of lead. My Chem teacher obtained it from one of the other professors just to show us (she was THE coolest chem teacher, but I digress)

I have also seen a quartz "glass" tube . It looked a lot like glass except the cut ends were clear (a comparable glass tube had the typical greenish end). I'm wondering, if this tube of quartz tube were to break, would it break along a plane, or would it break into rounded pieces like the glass tube?

[bill422]

A few more points about silicon and quartz. This is what I've picked up, since my research deals directly with putting chemicals on silicon using UV light that passes through a quartz window. (Is this relevant or what?). Quartz is nothing more than very pure glass. They both are made of silicon dioxide. Quartz, being pure, allows UV light to pass through. The impurities in ordinary glass don't allow UV light to pass. When quartz is broken it breaks like glass, no crystal structures--just shattered glass. (trust me, I have personal experience involving a $200 piece of quartz--SHIIIIIIIT!!!!). Pure silicon, in the electronics trade IS crystaline, similar to diamond (don't confuse pure silicon with quartz or glass which is silicon dioxide). Moreover, the silicon can be polished on one crystal face or another, or even diagonally to produce even more types of faces (these different faces go by names like silicon (111), silicon (001) or silicon (110). The silicon used almost exclusively in computer chips is Si (001).

The impurities in ordinary glass don't allow UV light to pass.
-Bill

The sunburned right arm I acquired while a passenger in a van driving eastward across Texas would question this assertion. If glass filters out all UV light why do we special (polarized IIRC) sunglasses for driving?



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Sue from El Paso
members.aol.com/majormd/index.html (http://members.aol.com/majormd/index.html)

[CatInHat]

IIRC (from a tour of the Steuben workshop in Corning a few years back), leaded crystal is softer than regular glass. That's why it can be cut. Of course, it's not soft enough to be able to tell the difference just by touching it; it's still pretty hard.

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The Cat In The Hat

[bill422]

Okay Fine! Let me amend that. The impurities in glass keep enough UV light from passing through that science types have to buy expensive quartz glass to do real UV work.

[torq]

why do we special (polarized IIRC) sunglasses for driving?

Polarized glasses are good for reducing the light reflected from shiny surfaces (like the rear window of the car in front of you, or the snow and ice covering the ground during winter in less fortunate climes, or bodies of water).

To the best of my knowledge, polarized filters don't do anything in particular to UV light which they don't also do to visible light.