If I ever take on glass engraving again, smack me...

[Joe De Medeiros]

Joe,

Did you actually just quote a Wiki article as reliable info?

no just for comparison, reread my edits i added after

[Dan Hintz]

Ah, the edit makes a lot more sense. It's a shame you're not closer as it sounds like a beer or two shared over the smelting pit would turn up some interesting conversations... I love peering into the science of what's going on rather than taking it as rote (unless it sounds "good enough" to not worry about).

[Scott Shepherd]

OOOOH, pick me, Steve, pick me! I have the smack to head down pat, mainly because I smack myself so often for taking on jobs I should have refused. I have a glass job coming up in the next week or so for a wedding planner friend of mine.

Belinda, I never had a big sister, but if I had one, I'd want her to be just like you

Always willing to lend a helping hand Good luck on your glass job, I hope it goes better than mine did. In all fairness, the large pieces went fine. It's the small text that killed me. Enough power to do what you need it to do to the glass and it blows out the text and makes it look fat (too fat). Knock the power down so the letters are sharp and crisp and it's not enough power to make the glass do what it needs to. It's gone now. They were very happy with all the items, so all in all, a happy customer, which is more important to me than money (Although I really like money).

[Michael Hunter]

Unfortunately, the Wiki thing appears to mix the properties of glass and stone.

Glass is supposed* to be amorphous (i.e. the same all the way through) and non-crystaline.

* Should be true of window, bottle and similar everyday water-clear glasses.
Very old glass (Roman era) appears to grow crystals within it (and is no longer clear).

I'm interested in Dan's explanation - I had previously assumed that it was simply thermal shock that cracked the surface and lifted chips.
Dan's theory of melting/resetting, plus Paul's microscopic observations together make a convincing argument as to what is really happening - and fits well with lower resolution/dither patterning giving the best practical results.

After an accident (on a customer's piece) I am aware that a 60W laser can easily melt glass - as well as shattering it!

[Belinda Barfield]

Belinda, I never had a big sister, but if I had one, I'd want her to be just like you

Thank you Steve. Considering how many times you have played "big brother" and defended me I believe we can officially adopt each other.

My glass job will be easy but I know who to call when I need to vent about it anyway. I only have to engrave a single large initial on a square vase (six of them), and the vases came from a local craft store. Knowing my luck though if I have a catastrophe the store will not have them in stock.

[Dan Hintz]

Glass is supposed* to be amorphous (i.e. the same all the way through) and non-crystaline.* Should be true of window, bottle and similar everyday water-clear glasses.

It is, unless it's tempered... at that point, all but the outer skin becomes ordered and properties change. Either way, hitting glass of any kind with the laser reforms the liquidous state (and removes any properties previously engineered in), and the quick cooling afforded by keeping it in the open air (rather than a temp-controlled oven) leads to a buildup of stresses at the shallow interface between glass and air. That's what pops...

Very old glass (Roman era) appears to grow crystals within it (and is no longer clear).

I'd have to see it to be sure, but I highly doubt anything is actively growing within the glass (crystal structure wise). It's most likely it was never clear to begin with. Old glass was not exactly pure and was used to allow light into a room, not necessarily a clear view of the outside world. Even during the days when optics were first being constructed, keeping the pot pure wasn't exactly easy... grains of still-cold sand (or other metals) would fall in as a block was being removed, it may have been smoothed over a hand-sawn log, etc.

I'm reminded of the old wive's tale that glass is a slow-flowing liquid, and "proof" is the panes of old glass that are thicker at the bottom than the top. The most likely explanation given these days is it was actually used for shadowing effect by the artist and installed with the thicker side down on purpose... proof of that is seen by a few old pieces being found with the thick side at the top. Flipping each piece top to bottom really throws the artistic effect off (I've seen a few Photoshopped images that show what the final image would look like if the pieces were randomly placed or all were used with the thick part at the top... it's a huge change in effect).

[TONG LI]

Ouch! Not only a party foul, but a machine cleanup and embarrassment factor, to boot...


EDIT: I find the timing of this thread somewhat amusing considering I'm currently working on my book's chapter on glass processing...

Clean up, yes, but no embarrassment. I displayed the broken beer bottle and told visitors, this laser machine can drink a beer and keep working, just like you, it is a working horse. Two blue Jeans kind of guys bought the machines at the show, lol.

It is not what you sell, it is how you sell it.

Tong

[Joe De Medeiros]

the quick cooling afforded by keeping it in the open air (rather than a temp-controlled oven) leads to a buildup of stresses at the shallow interface between glass and air. That's what pops...

When glass is cooled from a liquid state, it needs to hold at the anneal temperature for a set amount of time to release the stress, then slowly drop at 100deg/hr past the stress point (700 TO 800deg) depending on the glass, as Dan stated the temp drop is uncontrolled, and as it reaches the stress point it pops, or semi pops (missing letters).

I'm reminded of the old wive's tale that glass is a slow-flowing liquid, and "proof" is the panes of old glass that are thicker at the bottom than the top. The most likely explanation given these days is it was actually used for shadowing effect by the artist and installed with the thicker side down on purpose... proof of that is seen by a few old pieces being found with the thick side at the top. Flipping each piece top to bottom really throws the artistic effect off (I've seen a few Photoshopped images that show what the final image would look like if the pieces were randomly placed or all were used with the thick part at the top... it's a huge change in effect).

Most of that glass was hand rolled, so I'm sure it depends on the dirrection that it was rolled in, I get hand rolled glass from Bullseye, and it has a rolled edge that is thicker. Maybe it was just common practice to put it fat side down.

[Dan Hintz]

the temp drop is uncontrolled, and as it reaches the stress point it pops, or semi pops (missing letters).

It is possible to "pop" some of the stressed points that haven't passed the threshold themselves after cooling by rubbing with something a little hard (steel wool, paper crumpled into a tight ball, etc.). There should be few points that don't relieve themselves, and if you're unable send the remaining few over the edge by rubbing them, you need to increase your power.

[Joe De Medeiros]

It is possible to "pop" some of the stressed points that haven't passed the threshold themselves after cooling by rubbing with something a little hard (steel wool, paper crumpled into a tight ball, etc.). There should be few points that don't relieve themselves, and if you're unable send the remaining few over the edge by rubbing them, you need to increase your power.

Luckily I don't do signs. but here are some of my Dichroic glass jewelry

[Robert Walters]

glass "etching" comes from localized surface tension. The laser heats up the surface of the glass to the melting (flow) point...

Is this "reflow" the same process for granite and marble as well?

I swear someone at one time mentioned "micro fracture", but I couldn't tell you the context if I tried. Something to do with the percentage of quartz crystal in the medium.

[Dan Hintz]

Gorgeous work, Joe... now I really wish you were closer. If you ever make a trip south of the border, let me know and I'll have the beers waiting.



Robert, Doubtful, but I'll have to think about what process is actually going on there...

[Makrel Johnson]

Because I didn't find any recent rotary stemless frustration - I'll tag on the end of this...plus it was nice to know it's not just me.

Just (finally) completed a job of stemless red wine glasses and it was the first (and maybe last) order of this type. These were colored glass made by Lenox and not only were a pain, but stunk up the place while engraving.

Fun part was that the 'typical' glass speed & power settings turned out to be equivalent to a death ray as it cracked 2 right off the bat. This was after tinkering for what seemed like days trying to get the Epi rotary to not swing the glass like a club when it rotated, courtesy of the rotary clamp, and find a way to level the surface enough to not have ghosting. On these, if the focus was out even slightly - the lettering would only shoe an outline and not fill. Weird. It would also lighten up as if the power dropped at certain sections of the script.

Anyway - Scott I feel your pain...BUT - I now know more now than I did when I started, and have spares to work on a reusable jig with. I hope that it makes these things less of a MASSIVE Pain.

Live & Learn.

-Mak

[Don Corbeil]

Flat glass is fine. Can do multiples pretty easily.
It's the drinking glasses that bother me - the time it takes to do them individually in the rotary requires large blocks of time. The thought of starting in on a few cases of pint glasses just makes me wince. It also ties up the machine so that I can't easily do flat work without removing the rotary and changing settings back.

[Bill Cunningham]

The only piece of glass that has ever caused me any problem was a very thin ridel(sp)? Wine glass that a customer wanted a photograph on. The photo came out perfect, but after I gave it a light wipe, it literally fell apart in my hands. So, I redid it on a heavier glass and no problem. There is picture of this job posted here someplace. Other than that, every piece of glass I have put I my laser that started with a good photo, has come out perfect .. Ya I'm bragging For text, use 600 dpi