Laser engraving on glass

[Tech_Marco]

I found a good article talking about engraving on glass. Here is the link:
http://www.engraversjournal.com/article ... index.html

I posted the completed articale below just in case it went away. Pictues of the original post won't be able to show here though.


There is always lots of talk at tradeshows and workshops about using a CO2 laser to mark glass. Sometimes the information is accurate and sometimes salespeople get a little carried away with their claims. This article is intended to share my nearly 15 years of experience as to what lasers will and will not do when working with glass. For those new to lasers, I’ll also throw in some tips about how to get the best results.
Each type of laser works at a specific frequency. That frequency range (wavelengths of the light produced by the laser) determines what the laser will and will not cut. A YAG laser may operate identically to a CO2 but because the frequency is different, it will affect materials in a completely different way. There are any number of different frequencies used by lasers and each determines what it does best.
For instance, the frequency of a low power CO2 laser like that used in the engraving industry prevents it from marking metal at least in most cases. It does very well with most “natural” materials such as cork and wood and it does well with acrylic and some plastics. Others lasers, such as a YAG or Vanadate perform poorly with these materials but have no problem marking many metals.
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Figure 2: In this exaggerated illustration you can see the difference between the smooth edge of sandblasted vs. the ragged edge of lasered glass. Figure 3: When engraving on glass you may get an inconsistent color if the lens isn’t correctly focused as seen here in the Christmas tree.



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Because glass is a “natural” material (that is, its primary elements come from nature), one would think a CO2 laser would engrave glass pretty well. Ironically, it really does not engrave glass at all but it will mark it and with that contradiction, we will begin to understand how a CO2 laser beam interacts with glass.
In the most basic of terms, glass is made by heating silicon (sand). When heated hot enough, the silicon melts and becomes a thick, gooey fluid. This can then be poured into molds or blown by hand. As the molten glass cools, it remains transparent. To add strength or color to the glass, other elements are added. These are often metals such as lead, zinc, cobalt or even pure gold. In the case of lead, a considerable amount can be added to glass without affecting the transparency of the glass as you can see in 24% leaded crystal. 24 Karat gold on the other hand, produces a unique colored glass called cranberry.
Understanding this part of the process is important to laser users because, as all experienced laser users know, you cannot mark metal with a CO2 laser and that is just as true when the metal is molten (in glass) as it is when metal is in a sheet form.
So, thus far, we have learned that CO2 lasers can’t engrave glass, nor can they deal with the metal content of glass, yet we have all seen glass items marked by lasers. What gives?
This is really not as much a paradox as it might appear. Remember how glass is made? Silicon is heated to 2000‹ or more until it becomes molten. It is then removed from its oven and manipulated in some way. This introduces two things into the glass beyond its basic elements: air and moisture.
It’s the air and moisture that’s trapped within the glass that makes laser marking possible with a CO2 laser.
As a laser beam strikes the glass, it heats up the glass elements including the silicon and any metal content but neither of these elements will react to the relatively low heat and frequency of a CO2 laser. What does react is the air and moisture trapped between the elements of silica and metal. Both water and air expand when heated. Because the glass itself is relatively rigid, when molecules within the glass are heated until they expand, something has to give to allow for the expansion. This results in microscopic fractures in the glass, especially at the surface of the glass. It is this chipping or fracturing that we see as engraving.

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Figure 4: This image of a lightly sandblasted item shows the precision and consistency offered by an experienced sandblaster.
Figure 5: When working on a glass with complex curves, you can level the primary engraving surface using an inexpensive level.



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If the glass were actually being engraved, it would end up looking much like plastic after it has been engraved. There would be material actually missing—vaporized by the heat of the laser. But that isn’t what happens. If you look closely at the “engraved” surface of glass, you will see tiny “shards” of glass laying on the surface. Look even closer and you can see the microscopic fractures within the glass.
This is why it’s nearly impossible to get a laser “engraved” image to have a “high definition” image. With the glass fracturing first this way and then that, there’s no way to control the reaction with any great precision. It’s like a million tiny explosions taking place along the edge of the mark. When viewed normally, the mark might appear relatively straight and clean but a closer look will reveal what really takes place.
For years, there has been a debate as to how the quality of a lasered product compares to a sandblasted product. Especially laser salespeople sometimes wear a special pair of blinders preventing them from seeing the tremendous difference in quality between the two.
Glass, when both are done properly, sandblasting or rotary engraving for that matter, will always be superior in quality to a laser. A sandblasted line can easily be straight and clean where a lasered line is not.
Therefore, it should be clear that lasers are not the best way to mark glass—right? Wrong. In fact, in many applications, lasers are ideal for marking glass. They are less expensive, faster, more flexible and more forgiving than either mechanical engraving or sandblasting. Allow me to make a few comparisons.

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Figure 6: Curvatures and imperfections in glass can often be handled using your laser’s depth of field. With a depth of field of 1/4" I focus my lens at 1/8" down from the highest point to be engraved. Figure 7: If you have little shards of glass that result from your laser, you can use a nail brush to remove them.



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Mechanical Engraving
Mechanical engraving of glass is not as commonly done as in our industry as it once was. Certainly, there are still machines capable of it and engravers who are very good at it. In essence, it requires a rotary engraving machine capable of holding (clamping) glass products within a specially constructed container that allows a water-based coolant to be applied to the cutter while engraving is taking place. The process is computer controlled so it does not require any kind of mask or screen but it’s time consuming and requires the product to be firmly clamped. Products with non-cylindrical shapes, handles, spouts or indentations can be a major problem. Fragile items can also be a concern. Mechanical engraving however, when done properly, produces a beautifully engraved product.
Sandblasting
Sometimes referred to as “sand carving,” sandblasting has been used forever to mark glass. It requires a stream of abrasive (usually this really isn’t silica sand but man made abrasive sand materials such as aluminum oxide under air pressure.) The engraving results from placing a stencil-like mask over the item to be engraved covering up any areas of the product that one doesn’t want marked. This allows the abrasive to blast through the holes in the mask and erode away the glass. When done properly, the results of sandblasting are stunning. The cut can be very, very shallow (called “frosting” the glass) to very deep. It can even be done in layers giving a three-dimensional effect within the glass. Some of these pieces can sell for thousands of dollars and easily qualify as an art form.
The problem is that each step and each piece requires a separate stencil or mask. This takes time to make and apply and does offer some restrictions as to how highly detailed an image can be. Although the stencil artwork can be generated with a computer, the masks themselves are usually either made with a UV exposure unit in a dark room or cut into thin brass sheets which even further limits the detail possible but allows the same mask to be used multiple times. The so-called “photomasks” or “photo-resists” require making a film positive to expose the photo stencil materials with. This process is dirty involving sand (abrasive), air pressure (air compressor) and a blasting cabinet to contain the mess. Keeping the abrasive inside the cabinet is impossible. Therefore, this process is commonly used for very high quality products, leaded crystal, short runs or with products where either multiple layers or deep engraving is desired.
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Figure 8: To add color to your glass engraving try Rub-n-Buff, available in 18 colors from most craft suppliers. Figure 9: When engraving steins, wine glasses, vases, wine bottles and the like, using a rotary attachment will make the task much easier.

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Laser Engraving
Marking glass with a laser is fast, easy, clean and doesn’t require a mask, film work or water. Most important, it doesn’t require the product to be clamped down since there is no physical contact with the product being engraved. If it can be positioned properly, it can be lasered. Lasered glass can include almost anything you can generate on a computer screen. Text, photographs and artwork can all be lasered. Lasers are especially good at turning out highly detailed artwork—artwork that would be too complex for a mask or on products that have complex surfaces that wouldn’t allow a mask to be applied.
Lasers have some real limitations. One is the type of glass that can be lasered. Any glass with metal in it is suspect. Although most colored glass will laser, there are times when so much metal has been added that it causes the laser to “skip” over an area of the glass. No matter how many times you laser it, those spots just will not mark.
Leaded crystal is hopeless. Leaded crystal has so much metal in it, trying to mark it with a laser is nearly hopeless. Don’t waste your time.
In fact, the best glass by far for laser engraving is the cheapest glass you can lay your hands on. This really inexpensive glass has little or no metal content and usually marks very well. Those imported Christmas ornaments, wine glasses from Wal-Mart, inexpensive plates and platters from glass supply houses and beer mugs from the dollar store almost always laser like a dream.
Why would your customer accept second quality (laser engraving) over sandblasting? Several reasons:
First is time. In the same time it takes to make a stencil for a blasted item, you could probably laser a dozen of them and have them out the door. Time is money. A real-life example is an order to engrave both sides of 1,000 hand blown Christmas ornaments. Blasting would have required two stencils over a sphere, blasting and cleanup. By using the laser, I was able to complete the order in 1/10th the time. With no preparation, the ornaments came out of the box, into the laser and back into the box. A matter of three minutes each. The only overhead was the labor of taking them out of the box and putting them back.
Second is cost. Except for the few who are looking for and willing to pay for first quality sandblasting, most people are more than happy to save money and live with the quality of a mark made with a laser. Truthfully, until you point out the differences, most people won’t know there is a difference.
Third is complexity. If you’re working with a wine glass or beer stein and want to put an image on both sides, you’ll probably have to make and apply two masks for blasting. If the shape of the product is very complex, you may have difficulty applying the mask without distorting the image. Using a laser, there are no masks. Just flip the item over and tell the laser to “Start.” As for complex curvature, as long as you’re within the limits discussed, it doesn’t matter.
Fourth is price. Making stencils and applying them takes a lot of time and costs money. Blasting the product and cleaning it up afterwards also takes time. Waste can be a major factor since bits of the stencil can be “blasted” or blown off during the blasting process and ruin the item. Waste with a lasered product is less likely and there is minimal setup time beyond generating the artwork. A sandblasted beer stein may look a lot better than a lasered one but how much is a customer willing to pay?
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Figure 10A & 10B: Do a “test” engraving on cardboard or heavy paper to insure your engraving goes where you want it. Then visually position the item over the test to achieve the final position.

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How to Laser Glass
Lasers are easiest to use on a flat surface. That is, they require a specific focal length or distance between the lens and the surface being marked. When the item to be marked is curved, a laser is very limited. There are a couple of tricks that allow some flexibility here but even then, the distance around the curvature of an item is severely limited.
When engraving steins, wine glasses, vases, wine bottles and the like, using a rotary attachment can make the task much easier but still variations in curvature and complex angles can be a concern. Also, handles on steins can interfere since they may protrude enough to actually bang against the lens assembly as the cup spins around in the rotary attachment.
Because lasers use a lens, they have something called “depth of field.” This is the distance between the nearest point of sharp focus and the farthest point. Outside this narrow range the laser spot goes out of focus and consequently the image becomes blurry. Anything within the lens depth of field will laser engrave on glass and appear to be in focus. This gives us the ability to work with products that have a slight curvature to them such as plates, wine glasses, etc. Although not ideal, we can focus the lenses at the two extremes of this focal length and allow the image to wrap around the object with little or no visible distortion. To do this, you will need to determine the distance your lens will allow you to drop out before distortion becomes visible. On a 2-1/2" lens, that is about 1/4". Therefore, if we identify a point 1/4" from the highest point of the object being engraved and focus half-way between those two points, the center will be in perfect focus while the top and lowest point will only be slightly out of focus. That’s not enough to see in glass with the naked eye.
Generally, I “engrave” glass hot and fast. On a 25 watt laser, I laser glass at 100% power and 100% speed. I want the laser to strike the glass, do its thing and move on. Not everyone agrees with this technique and to them I say, “If something else works better for you, do it. This is what works for me.” I also recommend turning down the PPI of the laser to about 300. This better spaces the heat out over the glass and will reduce the tendency for the glass to create shards (tiny specks of glass caused by severe fractures). I want my engravings to be shard free, although that’s easier said than done since every piece of glass can behave differently.
Many engravers like to place a layer of wet newspaper or paper towel over the glass to keep the heat from radiating as much. This too is intended to reduce the number of shards. I have mixed feelings about this technique. Sometimes it works very well while other times, it doesn’t. I find it very helpful when marking wine bottles or using higher power lasers. To try this technique, wrap or cover the item with a piece of newspaper and then spray liberally with water from a spray bottle (add a drop or two of dishwashing liquid to make the water absorb into the paper faster and more evenly). If the paper begins to dry out while engraving, just stop the laser and spray again but do not physically touch the item being marked.
The objective of lasered glass is to leave a frosty mark that is easy to see and smooth to the touch. Running too hot or at too many pulses per inch (ppi) will create excessive heat and depth to the mark. Although this may sound advantageous, it really isn’t because of the shards and uneven engraving that will result.

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Figure 11: Holding jigs can be improvised using all sorts of unusual items. This glass barometer is balanced between two rolls of tape. Figure 12: Use caution when using a cylindrical attachment to engrave items with spouts and handles as they can cause damage to your lens when the item rotates.

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After engraving, the glass may need to be cleaned up a bit. If shards are present, use a nail brush to remove them. Cleaning the glass will usually be necessary. I use rubbing alcohol in a spray bottle and a Bounty paper towel. The alcohol cleans oils off and dries quickly.
Sometimes it’s nice to add some color to engraving. Other times, the nature of the glass may allow it to be marked but the mark is difficult to see or the image is so highly detailed, the detail is lost due to reflections off the shiny glass surface. One way to solve this problem is to add Rub-n-Buff. Rub-n-Buff is available in 18 colors from most craft suppliers. It comes in a small tube and is applied best with the tip of a finger. Very little is needed since this stuff goes a long way. Rub-n-Buff is not intended for glass so the instructions on the tube are not appropriate. The instructions say “allow to dry.” Do not do that. Apply the Rub-n-Buff and remove excess prior to drying. Otherwise, you may never get this stuff off. Once dry, it is relatively permanent.
One of the biggest problems with lasering glass is that glass often varies in size and shape. Even two wine glasses that look just alike may actually be quite different in size and shape. This affords a considerable challenge to engraving them. For me, the magic button for engraving glass lies in two things: One is something I call “Sample Engravings” and the other is the ability to create simple jigs or fixtures to hold the glass in place while being engraved.
What I mean by “Sample Engraving” is simple. Place a sheet of tag board, paper or cardboard on the stage of your laser. Adjust the focus and actually engrave the image on the cardboard. Now, all that you have to do to be sure the glass is positioned “just right” is to position the glass over the engraving so the engraving on the cardboard is positioned exactly where you want it. Then, re-focus and engrave the job again. No offense but it’s pretty hard to mess up a piece of glass using this method.
The other key is the building of “quickie jigs” or fixtures to hold things like wine glasses or other odd-shaped items so they don’t accidentally move while being engraved. I usually do an “idiot engraving,” then build my jig around it. This not only holds the piece in place, but it allows me to do multiple pieces quickly and easily. Items that vary slightly in size are easily accommodated because of the sample engraving on the cardboard underneath. The fact that items don’t have to be clamped in place is where a laser really pays off in the glass market. Marble trophy bases make great temporary jigs. For round products, consider a roll of tape. I keep several rolls of masking tape around. One has 1/2" wide tape, another 3/4", another 1" and so on. By adding these 3" hubs of tape together, I can create a wide variety of fixtures to hold oddly shaped items.
Summary
Is laser engraving as good as sandblasting? Usually not but it has many other advantages over blasting making it a preferred method by many people (particularly laser owners).
What kind of glass should you use? Cheap glass works best. Avoid anything with a lead content such as crystal.
Key to good engraving: Keep the glass as cool as possible. Don’t over engrave. Use wet towels and space out the ppi setting as much as possible.
If you aren’t already engraving glass, I hope you will give it a try. It really is easy, especially flat pieces. People love the look of glass. It speaks of quality and class. It carries with it something of permanence and prestige and affords a high profit margin. Although some glass certainly works better than others, most inexpensive glass does very well. Even brand name stemware like Libby does quite well and because those brand-named products are very consistent, once you have engraved a few pieces, you will know exactly how it will react each and every time.
Glass. There is something about it that everyone loves. Give it a try.

[twehr]

Good article. Thanks, Marco!

[baccus61]

Yep. great article. I will be reading this a few times to absorb it.
Thanks Marco.