This is 5% doing, and 95% mucking around and finding out. You *will* have failures, and that’s both ok and normal. Get yourself some waste material, the cheap stuff, and see what happens.
When looking to do some engraving on glass or clear acrylic, a masking of some sort is needed for smaller lasers. There are a bunch of different options, paper, tape, marker, paints, etc. Big considerations for selecting one of these include ease of application, quality of engraving, and cleanup. For either material, you need an even opaque coating to get the most detail from the laser. Spots where the coating is not even will result in a splotchy appearance. And if you want to have a single coating method across both materials, then you have to eliminate most anything that requires chemicals (acrylic + chemicals = bad). Two of the most talked about products are black gesso and tempera paint. Each are readily available, can be applied in different ways, and wash off with water. But what I wanted to know is, is one of them actually better than the other? Anecdotal evidence supports them both, but nothing definite that I’ve been able to find. So that is what I set out to prove to myself.
For the quick version, just down to the Summary Chart.
Cost
This is a little tricky, as prices on these two items do fluctuate depending on brand, buying location, product size etc. I tried to take an average based on 10 listings on the big online store. What I came up with wasn’t too surprising. Black gesso came in costing ~33x more than tempera paint. That’s quite a difference! But, cheaper is not always better, so while tempera paint gets a one up on gesso on price, that’s not the end of the story.
Application
Both of these materials can be applied in a variety of ways. Brush, obviously. But also airbrush, if diluted. The gesso I picked up, the one most often pointed to in various groups, has a consistency somewhere between pudding and cold creme, closer to pudding. Tempera is roughly around Elmers Glue, thick. For the brush challenged, (me), getting these onto a completely smooth surface like glass or acrylic sheet in something that approaches perfectly even is highly difficult. Brush marks galore, hills and valleys, etc. Perhaps this is easy for some other folks? Looking at the instructions on the gesso, I’m guessing not so much, as the maker specifically mentions to sand each coat before applying the next, suggesting they know it’s not going to go on smoothly. Anyhow, I found that diluting both with water helped out substantially. Gesso, as a 50/50 mix with water, went on much more smoothly and could be airbrushed. The tempera paint was good to go at 25% water.
With the ratios worked out, I mixed up 4 drams each, and set to work coating some glass. Brushing turned out to be better with gesso. Two coats and there was a relatively even opaque coating on my test pieces. The tempera wanted to be a pain. Every time I went to add a coat, the coat underneath would become semi liquid again and smear around. Even letting a piece sit overnight, to ensure that it was fully dry, same problem. One up for gesso. When it came to the airbrush, both were equal. 6 coats each did the trick, dry times were equivalent, and total prep time was about 30min each. Cleaning out the airbrush was easy peasy, and neither paint caused a clog. Both also had some of the mix leftover.
Speed/Power Test
To see how each material would affect etching on glass, if at all, I created a variation on the typical speed/power test. Instead of little squares, I brought in a small image. A photo might have been better in some ways, but the image I used had enough fine details and opaque areas that I figured it would be a good example to use. Each image was 20mm square, or roughly ¾” in size. The idea here, as usual, is to find the highest quality at the highest speed and lowest power usage. As such, I set the speed parameters to 1-6kmm/m and power 20-80%. What I was looking for was the most crisp lines, vibrance, and completeness of the engraving. If all you’re looking to do is big bulky letters and logos, vibrance is the only bit you’re going to be interested in.
The gesso got to go first. At the 1000mm/m speed, the engravings were very distinct all the way down to the 30% power range. But, they also were hazy, with the lines being a bit blurred and details wiped away. At the other end of the scale, 6000mm/m, the images were rather faint. The images with the highest power were more faint than the images from the 1000mm/m speed at 20% power. After much looking back and forth, the one I thought looked best was the 4000mm/m 70% power image, and it was a close call with some of the 3000mm/m images.
Now for the tempera. Frankly, I wasn’t expecting much difference. And I was wrong. Across the board, the images were far more vibrant, well defined, and complete. I put the top mark at 6000mm/m with a 60% power setting, but it’s clear that if my machine was stable enough to push all the way up to the 10k limit, engravings would probably still be possible with full power. That’s saying something.
Cleanup
As previously mentioned, both tempera and gesso can clean up with water. Gesso particularly likes hot water to get things going, and comes off like dried latex house paint. If you’ve ever had to clean a paint tray with dried latex paint in it, you get the idea, it comes off like a thin skin. That does bring up the question of if you want to have that going down your drain, as it can create clogs/blockages. It’s worth mentioning that a brush was needed to get the gesso out of some of the crevices. This, like hot water, could be a problem for acrylic material as it could cause damage.
The tempera paint, well, it just dissolved away in seconds. Cold water, hot water, tempera didn’t care, it just washed away. Okay then, that works very well.
Summary
All bias and anecdotes aside, the data doesn’t lie. Tempera paint has shown itself to be equal to or superior to black gesso in every measure except putting it on by brush. It’s cheaper, renders a significantly better engraving on glass at high speed, and getting it off glass is just a sink away.
Summary Chart
| Black Gesso | Tempera Paint | |
|---|---|---|
| Cost | $0.33/mL | $0.01/mL |
| Consistency Equivalent | Pudding | Elmer’s Glue |
| Brush Application | Difficult | Difficult |
| Dilution with Water | 50/50 | 25% Water |
| Diluted Brush Application | Good, 2 even coats | Poor |
| Diluted Airbrush Application | Good, 6 coats, 30 min | Good, 6 coats, 30min |
| Best Engraving Setting | 4000mm/m @ 70% | 6000mm/m @ 60% |
| Water Cleanup | Comes off like a skin, easier with hot water | Dissolves away |