Why and How Glass Breaks

I spent far too much time last week contemplating why we apply pressure on the opposite side of a score to break glass, and had to do a few more ‘GLASS IS NOT A LIQUID’ screeds. I decided this was worth a blog post. THE SCIENCE OF WHY AND HOW GLASS BREAKS.

1 - What Glass Is!

Please, repeat after me, GLASS (clap emoji) IS NOT (clap emoji) A LIQUID (clap emoji)! In grade school we learned that there are three (or four) phases of matter, right? Bad news, Mrs. Thompson lied to you. Ok, didn’t lie, just grossly oversimplified. 4th graders probably won’t understand the degradation of neutrons.

Solid – A uniform and dense configuration of tightly-bound molecules gives us rock, wood, ham sandwich, our skulls, etc.

Liquid – Less tightly bound and it’ll slosh around in your whiskey glass

Gas – Freer still, and it’ll fill up a room or otherwise expand to fill a container.

You might have also gotten to Plasma, which is the bright flash of lightning during a storm that ruins your day if it hits you.

Glass doesn’t fit neatly into any of these, so over the years various stories have developed. It must be a liquid, it must be a solid, etc. The truth is there are a lot more than 3 or 4 phases of matter, and glass is in a category of its own. It’s an ‘amorphous solid’ but that puts it in its own category along with degraded neutrons and other more unique things.

1b – Glass is NOT A LIQUID, pt 2.

The amount of time it would take for a glass object to ‘melt’ is something like ‘the age of the universe’. There is a very common misconception that old window glass is thicker on the bottom because it has flowed that way. If that were true, we wouldn’t have glass objects from Ancient Egypt and Rome. People wouldn’t have heirloom glasses older than grandma’s house with the weird windows.

The reason there is a thicker part to the window is that older methods of making glass sheets (e.g. the crown and cylinder methods) produced a sheet of irregular thickness. It’s some kind of ‘common sense’ to put the thicker part down for stability, but it’s not a rule and contrary examples exist.

1c – Wait, you didn’t really explain what glass is

Glass is solid, in the common sense that it’s not a liquid, gas, or plasma at room temperature (on earth, etc etc). But its structure is very disorganized, which is why it has similarities to a liquid. If its molecules were more orderly you would have "devitrified glass" ("de-glassed glass"), which qualifies as a ceramic.

Because of that attribute, a sheet of glass has strains within in. Various internal pulls and pushes and twists, so to speak. When glass is annealed a great deal of the stresses are able to resolve themselves, but not all. This is also why glass doesn’t have a grain like wood.

[Update: I thought it might be good to note that while current science identifies glass as a separate phase of matter, earlier academic documents would refer to it as an "amorphous solid" and, going earlier still, as a liquid. I believe that was accurate (for the time) using a very scientific definition of liquid as opposed to the common understanding. However, "Glass is NOT a liquid!" is much more concise than "Glass does NOT melt into new shapes over the course of one or two hundred years because it is not a liquid in the layperson's meaning of the term which has been superseded anyway by scientific progress and the inexorable march of time!"]

2 – How glass breaks, unsupervised

As is usually the case, force likes to take the Path of Least Resistance through glass. If a substance has a grain, like wood, you may be able to guess how it would split if hit with a general force. But in the case of glass, there is no grain and the stresses are mostly invisible[0]. As a result if you hit a piece of glass the fracture lines will follow the path of least resistance taking advantage of these faults and weaknesses until it reaches the edge of the sheet.

It's interesting to note that research (source below) shows you can apply less force than is needed to run a score if you apply it for a longer amount of time, even as much as 50% less. The authors note that if glass is under strain, it REALLY needs to be protected or minor damage may destroy it.

3 – How glass breaks, supervised

All we are doing when we break a piece of glass is coaxing the path of least resistance to be a route we want. I’d say ‘choose’ but if you’ve spent more than five minutes breaking glass you know it’s more of a coaxing and/or tearful pleading.

A typical modern glass cutter has either a steel (cheap) or tungsten carbide (much better) wheel that is shaped like a chisel. Incidentally the angle of that ‘chisel’ MAY have an influence on the cutting. I had never ever heard of this before until someone mentioned it (from a Wikipedia article). That article refers to a book on automotive glass and says a more sharply angled wheel cuts thicker glass better, from 120° - 154°. I don't work with industrial glasses so maybe that is common knowledge to them, but 154° is oddly specific to me.

Bob Beranek; Ann Schuelke (1 August 2011). The Complete Guide to Auto Glass Installation. AuthorHouse. p. 336.

Both steel and tungsten carbide are harder than glass, and so they are able to bite into the surface of a sheet. Carbide is just much harder than steel, so it is more durable. Usually steel wheels are found on cheap cutters you get at a hardware store. Carbide wheels are what you would expect from a specialty store and most anything directed at the stained glass market. Carbide cutting wheels also almost always have an oil reservoir in the handle.

Why oil? Its more to do with the tool than the glass. The stress of scoring glass builds up heat, which could eventually mess with the tempering of the metal wheel. It won’t generally be enough to cause thermal shock in the glass, but little glass particles are also caught in the deposited oil and that’s a good thing.

Fun fact, researchers (source below) discovered that the pressure applied to your cutter has an S-shaped effect on the force required to run the score (that is, break the glass). Once you reach the minimum force needed, adding pressure (pushing down your cutter harder) actually increases the torque needed to break it... until you blow past that zone in which case more pressure makes it easier again.

3b – Bringing it together

When we ‘cut’ glass we are actually introducing a known fault line into the glass in the hope and expectation that, when flexed, the path of least resistance will follow our score. And we all know that, sometimes, it doesn’t. Maybe a curve was too sharp and the PoLR was a straight line. Maybe a seed (bubble) was present that coaxed the line askew at a point, and then the PoLR is to follow the new angle instead of bending back to the score. Those invisible stresses can also be at fault, because glass is a temperamental mistress and lives off our tears. Wait, what? </Bitter Glazer>

4 – So, why DO we press from the opposite side of the cut.

It bothered me that it made sense in my mind, but I couldn’t explain it in words. I actively chewed on this for the better part of a week, thinking in terms like a race track. If you’ve seen one, you probably noticed that the starting line is staggered, and the closer to the center you are the further back you begin. This is because the center ring is smaller, and shorter, than the outer rings so that racer has to be further back to get the same distance. I was trying to leverage that rationale to explain breaking a flat sheet of glass and it didn’t quite work until I stopped thinking about a donut-shape.

 Not my best drawing, but showing that running pliers (or your thumbs) applying pressure on the cut effectively applies pressure to squeeze the score line. You might get the glass to break but it will be uncontrolled and unguided and probably isn't following your score. Note that the score is magnified A LOT in this drawing.

Another little sketch showing a greatly magnified score under no pressure, some sort of light pressure, and finally breaking pressure.

Incidentally this CMoG blog post has an even better graphic of this I found two days later, using foam.

5 – Bonus: Why your hand position is so important when cutting

I originally learned to cut glass by holding my cutter like a pencil. It's not quite that relaxed or the wheel wouldn't be able to contact the glass, but similar. I also noticed my accuracy was never where I wanted it to be, and I was always grinding pieces, finding new ways they didn't fit my pattern, grinding other areas of the piece, and eventually throwing it into scrap. After I began teaching with Molly I learned from her the "peace sign" grip that is pretty universally called for in professional-grade books.

  

My hand holding my favorite  cutter in a comically relaxed pencil grip, a much more likely pencil grip, and finally a 'peace sign' grip. The camera was not straight on, but you can still clearly see how the first two are further off-center than the last.

 I haven't found solid research into this, but I strongly believe that when you hold your cutter off-center, the score itself (and the associated "microfractures" if you believe that idea) are also at an angle. Least resistance being what it is, I suspect it would be more like a J-shaped curve than an actual diagonal, but you end up with a cut that is not perpendicular to the surface of the glass, what I know as a shelf [1]. Shelves are ridiculously sharp and seem to disappear if you even bring them near a grinder. I think that this is why my accuracy was poor for a long time;

You can see the 'shelf' on this piece, right above the little bump. It took me seven tries to hold a cutter wrong and get this piece

Holding the cutter off-center isn't going to give you a shelf every single time, or I would never have lasted this long in the hobby, but for me there was a major reduction in shelves and an increase in accuracy when I learned to use the better grip. Shelves are ridiculously sharp and will cut through your copper foil and your fingers effortlessly, not to mention (again) mess with the size of your piece. Probably a half-dozen times I have cut myself on a shelf-y piece and watched the delay as my skin realizes it's been cut, recovers from its shock for a second, and THEN starts bleeding. 

Allegedly neurosurgeons use obsidian scalpels because glass can be 100x sharper than steel. I believe it can be sharper than steel but about a year ago when I tried to investigate this, it seemed these scalpels were only an experiment.

6 - Really Cool Bonus: Glass Heals

I had heard Molly talk about this fact one summer while we were teaching. I'll be honest, I didn't believe it. How can a glass score heal and become more difficult to run if you let it sit for a few days? While doing other research, however, I found an academic paper from 1937 (Info below) that tested and proved the concept.

Their research showed that if you scored glass and let it sit, the amount of force needed to run that score increases over time, maxing out around 16 days. By the third day it's done the vast majority of its 'healing', however. Their hypothesis is that over the course of days the innate stresses in the glass will work themselves out a bit and reduce the effectiveness of your work. 

In conclusion, glass has a number of odd behaviors that bring to mind the expression "wibbly wobbly timey wimey", we work with the glass and beg it to do what we want, and windows aren't melting because GLASS IS NOT A LIQUID.

--------------------


[0] - There are polarized filters you can look through that will show the strains, but I don't THINK they are practical for non-scientists and I'm pretty sure they aren't the same thing that lets you tell which side of float glass hit the tin.

[1] - I am finally reaching that age where I can't quite remember whether I came up with this term myself or if I learned it from Norma/Danielle at GlassLink a decade+ ago.


Sources:

https://www.scientificamerican.com/article/fact-fiction-glass-liquid/

https://www.scientificamerican.com/article/is-glass-really-a-liquid/

https://en.wikipedia.org/wiki/Phase_(matter)

https://en.wikipedia.org/wiki/State_of_matter#Glass

https://io9.gizmodo.com/the-glass-is-a-liquid-myth-has-finally-been-destroyed-496190894

http://users.monash.edu.au/~ralphk/glass-cutting.html
(I'm not fully convinced on several concepts Mr. Klimek writes about, such as using spit to help run a score or the idea of microfractures, and we now know that glass is not a liquid but the anecdotal information is entertaining.)

"Concerning the Cutting of Glass" by MERTON W. JONES AND JULIAN M. BLAIR
Journal of Applied Physics 8, 627 (1937); https://doi.org/10.1063/1.1710352

https://blog.cmog.org/2015/06/03/part-2-why-does-glass-break/

Vitreous Paint Experiments (Malachite)

I bought a small bag of malachite chips to grind into pigment, to go into the William/Isolde scribal gift box. As I was having fun smashing it, it occurred to me that we do have greenish pigments, and I wondered if malachite might not compose one.

So, I smashed it and smashed it until I got cramping fingers and crossed eyes (Actually, not that long or difficult). I got out my handy 325 mesh sifter, as in theory that's the level Reusche grinds to, and sifted the malachite until I had a handy stash in an old spice bottle.

Because I am for scientific experimentation, I wanted to control everything except one variable. I used some Reusche clear glaze base so I could see what the malachite would do on it's own. I realize, I don't know the ratio of glass (or glass components) to oxides in pigments; One period text would say 2 parts glass to one part copper. I made four test chips to compare different ratios, 1:1 through 1:4.

I mixed them on an ad hoc glass palette with a muller. The muller, being glass, seemed the easiest to clean. I noticed how easily the pigment mixed, much like working with commercial (Reusche, Fusemaster, etc) paints and stains. The very fine mesh size seems to promote fluidity. I used a bit more water than I would for "real" but with this small quantity of paint it was rather difficult to get the right water content (I guess I could have tried to drip it off an eyelash or a cat's whisker, but neither myself nor Zod were willing to cooperate with that). The water won't change the performance of the paint, only how it handles on the brush, so having too much water shouldn't impact the results.

I weighed them on my mini digital scale to get the ratios. Although it does .1g increments, the floor seems to be .2 grams. That was what I used as a "unit", so the 1:1 chip is .2 grams of malachite to .2 grams of clear base. The 1:4 is .2 grams of clear base, and .8 grams of malachite. I wrote my name on the chips, for some reason the first thing that entered my head, to test the line work. I then smeared paint on the bottom block of each chip to show various values. I also marked the ratio at the top.

I then fired the chips on my standard vitreous paint schedule, which matches the range for the clear glaze base. The next morning I was fascinated to see significant change in pigment. What I was aiming for was something akin to "Grey green" pigment, a modern sample of which is here placed next to the chips:

The malachite DEFINITELY darkened. To my eye, it also seems more faintly blue than green. Azurite, a deep blue twin of malachite (both are copper (II) carbonate rocks) will turn into malachite when weathered, but nothing in my research shows the reverse. I did find that azurite when heated turns into copper (II) oxide, a black powder. That oxide is then used as a ceramic pigment, making blue as well as green, black, pink, red, and gray colors. I suspect that's what has come into play here, the malachite likely also forms the oxide which would account for the blueish hint and the darker color. I am not a chemist, however, and I couldn't easily find a reference to what happens when you heat malachite. [Edit: A friend reposted my link to get the attention of some chemistry-buff friends, and one Liz pointed me at this link. Malachite does turn into copper (II) oxide.]

I first picked up the 1:4 chip, and immediately noticed the paint flaking off onto my fingers. My fingertips were tinged grey/black/blue. I found, using a wooden skewer, that the 1:3 chip was also very easy to scratch paint from. The 1:2 I could leave some trace, but not much. The 1:1 completely resisted the stick like Reusche paints would. Looking at the reflected light, the 1:1 chip also looked much like a dozen other test chips I've made; the paint is completely glassy and adhered to the test chip. I fire my vitreous paints to the high end for that effect, so this is expected. The other three showed a rough, grainy texture I associate with previous experiments that had too rough an oxide.

             


                                                         


Lessons Learned:

• Yes, a randomly selected mineral MIGHT make a usable vitreous paint!
• Something near a 1:1 ratio is probably idea to bind the pigment to the glass, though 1:2 was also serviceable. My tests of the period formulas are 1:2, and were very similar.
• It would be wonderful to find out what actually goes into Clear Glaze. My normal secret trick is to check the EU vendors, who seem to list MSDS's that US vendors do not. Unfortunately Peli doesn't include one for Clear Glaze. Reusche gives them out if you make a special request in writing, whereas Peli just has them on their website. I rather suspect this is because the MSDS sheets rather give away the secrets. 

Two Mistakes: Hamsa

I had the idea to make a Hamsa. The idea persisted, and turned into a large flop. So, I remade it. Which turned into a second flop.

Enamels are difficult.

I ended up making nearly the same design, but pieced. I was mad and determined, at that point.  I started with a design I found online; I found it on several websites, in several ways and places. I couldn't determine an original creator, etc and chalked it up to a generic cultural motif. Having learned much from Estelle's scroll, I thought I planned it well. I used the more stable blue, and skipped the idea of using silver stain.

Attempt 1, I painted on the black line work, then added a mat of blue enamel. In the past, again for one of Estelle's badges, I got a phenomenal sapphire blue right off the bat. That gave me incorrect assumptions about how easy it can be to use!

This happened over a year ago, and Facebook isn't helping me work out the order. I know at one point I discovered I had two different blues on hand, with no recollection of the opaque one. So much so that I went back and found the order and made sure I had consciously ordered it (which I had).

The next piece, I kept adding blue trying to get the Sapphire hue I wanted. After a few coats the enamel turned opaque and off-color, a grayish tone to it. I still don't know why, but I decided I needed to get it right from the start.

The next attempt, after doing the line work and the enamel I decided I wanted to shade the background and went back, to add more black mat. That tanked the whole piece as well, as shown below.

Ugly and wrong color, above!

 My "mistake" in going back to vitreous paint. Guess blue enamel isn't as stable as I thought.

A closeup of the weirdness that ensued. I suppose too many layers of enamel flattened out and blurred the black lines beneath, like layers of glass can displace one another when fusing.

The final piece I successfully made and framed, to get a Hamsa out of my mind. I've not included a picture of the pattern to retain my blog's G-rating, as each piece was named with a different profanity, instead of the customary numbers or symbols.

The bottom right corner was 'bullsh*t' if I recall. It was easy to cut, that was just it's nickname.

Lessons Learned:

• Be sure you are using the glass paint you think you are using!
• Blue enamels are also touchy. All enamels are semi-evil.
• Weird things can happen if you apply them too thickly (?) or fire them too many times
• It looks like many layers can displace one another between firings.

Silver Stain Experiments Part 2

[Edit: No idea what this is still a draft. Publishing now, three years later!]

Phase 2 has actually turned up some successful blends!

Detailed below are tests I ran with pure silver, silver nitrate, copper sulfate, and silver sulfate as the active compounds. Binders included a new, "brand name" red ochre, yellow ochre, gum arabic, and brick dust. After doing a round of tests I came up with several new ideas, particularly where pure silver and copper sulfate are used, and had to do another set.

Some notes on the binders themselves:

Silver dust and gum arabic - Almost indistinguishable from pure silver dust. Film formed, gum arabic undoubtedly. Weird to see it plainly, but interesting to get a clear visual of the effects of gum arabic. I've noticed recently, when I added too much water to some matting paint, how there is a "thickness" to the water, as you stir the paint you can see some of the unmixed water jiggle and repel the paint until you force it to mix. I've not tried mixing paint with no gum arabic but I think I'll try it to confirm my thought that that is also the GA (rather than the paint). That may be what they refer to as "body" when talking about other binders mixed with water.

Brick dust - Probably needs to be even finer. Works well. I ground pot shards with a pestle and mortar, but I think I need to get it even finer. I'd love to get my hands on a ball mill to do this, but it's not in any shape period.

And a correction from "Part 1" in that I realized I'm slightly blind. As I began to tag and catalog my samples, I almost threw out a few pieces that didn't take. As I was getting ready to toss them I realized that the copper sulfate chip DID stain:

On two slides it's almost imperceptible (this picture above is not a very good example, but it shows up at least) but it was actually there. Because of this realization I tried a stronger mix of copper sulfate for this round. As detailed below in Sample 6 it didn't work out, but I have a good idea why not (temperature).


The stain samples prepared and drying:

A closer shot of the samples:

Silver nitrate and gum arabic made an incredibly pretty, deep orange color while drying. This continued to intensify as it dried, becoming a dark red

The samples after firing:

Sample 1 - Pure silver dust and gum arabic (1:6). Oddly enough, this did nothing. I suspect it takes a higher temperature to work. I know from Leap's book that pure silver leaf will leave a dark amber stain on glass, I can't imagine the powder not having the same effect. Later review of Leap's book indicated he fired his pure-silver tests at 1500°, a full 500° hotter than I did. I know what to try next, and may throw some more of sample 6 in with it.

Sample 2 - Silver nitrate and red ochre (this time from Vallejo pigments). Clear proof that there was an issue with the red ochre I bought before, this name brand sample didn't have the same hazy effect that the first sample left behind, ruining what results there might have been. Part of the sample is a little darker, but I suspect that may be related to how I used it (no blending, possibly imperfectly even surface allowing oxygen to get in, etc) rather than the mix. I consider this one a success!

Sample 3 - Silver nitrate and terracotta dust. A bit of research indicated most of the bricks before the 14th century would have been very similar to what we call terracotta. I bought a small planting pot, smashed it, and started grinding it into a powder. It seems to work quite well, actually. The one downside was that it must be ground very finely. Small "pinholes" are visible in the sample (close-up later in the post). This was caused by less well ground (larger) bits of the terracotta that inhibited the even spread of the silver nitrate. This also left behind a bit of hazing on the edge, but I again suspect it has to do with handling rather than the compound. I also consider this one a success.

Sample 4 - Silver nitrate and gum arabic. Oddly, this didn't work at all. No effect was left behind. Isenberg's book has a chapter on painting and a page on silver stain. It mentions that the "Reddish material" has minerals which pull the sodium out of the glass and allow the silver in. I find it hard to believe, but at the moment have no other explanation for this chip's complete lack of stain. The silver sulfate and copper sulfate chips, both mixed with gum arabic, had visible staining. Silver nitrate did not. The same book also states that "other silver salts" (presumably silver sulfate) are added to stain powders because nitrate is unpredictable and melts unevenly. I haven't seen that to be the case, yet, either.

Sample 5 - Silver nitrate and yellow ochre (vallejo). This mixture was twice as strong as the same combination I tested in "phase 1" and likewise is a much stronger color. This is 1 part silver nitrate to 3 parts ochre.

Sample 6 - Copper sulfate and gum arabic - This is a 1:6 strength mixture, better than what I had previously made. I discovered, just before pitching the last test chip, that copper sulfate actually had stained the glass. The compound was in such a small quantity that it was in the form of tiny specks. I mixed this more strongly and used more of it to try and get a more visible effect. I didn't get it, though again I got visible effects. I think this merits a higher temperature or longer soaking period.

Sample 7 - Silver dust and gum arabic, 1500°
Sample 8 - Silver foil pure, 1500°
Sample 9 - Copper sulfate and gum arabic, 1500°

 

A close-up of sample 5

 A close-up of sample 3

All 13 test chips I've fired so far

Lesson's Learned

Future Plans ("Phase 3")

The Arms of Baroness Laurencia of Carlisle, OL

Hello! It is a few minutes before midnight on the eve of Laurencia's elevation to the Order of the Laurel. I am scheduling this to go live early in the morning; I'll post it to Facebook mid-morning when it should be safe! Let me apologize in advance for the inevitable spelling and title screw-ups! Also please note you can click on pictures to enlarge most of them. -Brynn


I used to work with Laurencia and her husband, Eirikr inn havi four or so years ago. A few months ago he messaged me on Facebook to ask if I could do her device for her upcoming elevation.

The original image he provided

Knowing her elevation was to be at Ice Dragon in April, I sat it on it for bit. Too long, but ok in the end! I seem to recall being told Laurencia is part of the "14th century mafia" so I wanted to pick a design that was appropriate to her period. Looking at extant panels from France and England, I found several of an armiger's device set in "plain glazing" or a window of clear glass. Frequently we do quarries with more acutely angled points, diamonds like the suit on playing cards.

Like these! Image courtesy of the CVMA

In several windows of this time, however, I saw they were the 90-degree style diamonds. The more acute style seems much more prevalent in late-period windows, I'd like to dig into those changes sometime.

At one point I thought about setting her device in a ring of laurel leaves, but I found it very difficult to do so within a reasonably-sized window. I tried scaling my pattern up from 12 to 14 to 16 inches square, but I couldn't find proportions that were acceptable. Either the ring was too thin, or the device too small, or the combination awkwardly placed in the pattern. I eventually dropped that plan and just focused on her device.

I opted to used all mouthblown ("Handmade") glass for this. On hand I had sheets of Krosno (from Poland) and St. Just (from France). I had never mixed them, so this was the first opportunity I had to note the difference in their compositions:

The edge-on view gives it away. The green is caused by iron impurities, though a few sources claim some copper content can do that as well. Ensuring the iron is kept out, or adding selenium or lead, causes a clearer glass as with the other brand here. If I'm not mistaken, it's the St Just that is green.

The flashed glass I used, red flashed on clear, is of unknown provenance via THL Molly Sotherden, but despite hearing of it's existence I've yet to find machine-rolled flashed glass. Looking at the irregularities, I feel very confident it is mouthblown as well. I learned during this project that flashed glass can be a striking color (See Lessons Learned section below). Flashed glass has a thicker base layer of one color (usually clear) and a thinner layer of another. It is one solid sheet of glass with two distinct sides. Originally this was done because otherwise some colors were too dark to transmit light (red is the best example). A sheet made in the typical way would appear black. Giving a thin layer of color to a clear sheet allowed the red to shine through and remain strong enough to use. In the 14th century it became well-known that you could remove the colored layer and add extra detail without adding leadlines.

At this point in time, most glass cutting is done with a dividing iron (a red hot poker used to incite thermal shock in the glass to give it a crude shape), which can be seen in my post here and the video to which it links. The shape would be refined through a grozing iron (I now use a proper grozing iron made for me by THL Kendrick Cameron, but my original one can be seen here). About this period in time, diamond-tipped cutting tools are becoming known in Italy. I own an antique diamond-tipped cutter and used it a little for this project; unfortunately it is rather temperamental and I wasted half a sheet of clear glass trying to make it work. As a result the majority of the cuts use a modern carbide-wheel cutter to save glass/money.

My modern cutter, and my antique diamond-tipped cutter

Period sources talk of either abrading away the flashed side, or using acid to etch it away. The acid called for in period is generally not strong enough to etch glass unless it is of very poor quality (and hat tip to my Laurel, Mistress Kirsten Thorsteinsdottir, for that fact). Modernly we can use a stronger acid, power tools, hand abrading, or sandblasting. Sandblasting and acid end up with very similar results. Having tried all of these options in the past, and wanting a certain "hand abraded" look on "power tool" time tables, I abraded it using an engraving tool. It is less drastic than a dremel, and after firing I think it looks excellent.

The water bath is probably not period, but we prefer to avoid death by silicosis today! The picture on right is the piece before fire polishing.

I applied vitreous paint to the bird using a miniver brush, as called for in our primary source texts. Period formulas for vitreous paint call for crushed glass and mineral/metal oxides for color. Modern texts reference using glass fluxes and components, rather than actual glass, for making paint, however the oxides are still in use. I fired the three pieces, bird and two halves of the ermine section, in my modern electric kiln. My tests on proper period kilns are still forthcoming and, again, the glass is expensive to waste!

The central/bottom portion, after firing

The entire device, fired. I aligned the ermine spots to coincide with the lead line to try and mask it a little. Fully period cutting tools might have been able to get that steep inner angle cut out of one piece of glass. Might.

I cut the background of quarries and got stuck worrying about the next steps. Obviously lead came would be the period technique, and cast cames at that. I would have to use purchased milled cames, but my skillset with came is very rusty, and it's not as strong. In order to have a strong, lower-maintenance, and better looking end product I opted to use modern copper foil methods to assemble the window. That would be the biggest concession to modernity but I feel it was/is the right call.

The glass, all cut.

The glass, all foiled!

A pretty close-up 

The completed and framed panel

I put the completed panel in a wooden frame, which is called for in our primary sources. I added modern hanging hardware (eyelets and rustic copper chain) for Laurencia and Eirik's convenience. I hope that it arrived unbroken and that it is only one of many beautiful things she gets on the day of her elevation!


Lessons Learned


1) Diamond-tipped cutters are finicky!

2) From a discussion with Conor O'Ceallaigh, after fire polishing some non-yellowing lacquer could smooth out the texture further. I had several kinds on hand, but I wanted the abraded texture to remain.

3) I need more practice with framing stock. These pieces fit fine when I cut and test fit them, and held while I handled them after gluing. When I added the support brackets on the back, the joints popped a bit and gaps appear. OI!

4) This is only the second time I had an issue with a glass color striking on me, and the first was understandable as it was a green enamel. My sheet of red flashed was in two pieces; I knew one end was a prettier red and the other end was an odd brownish-purple color. I couldn't find the larger sheet with the better end, only a significant piece from the brownish-purple side. Remembering a funny story involving this piece two years ago (which I only told on Facebook, I'll add it here later today), I checked it against two different light sources. Sure enough, backlit by an incandescent bulb, it was beautiful ruby. Against fluorescent or sunlight, purple-brown.

Color being what it is between devices and cameras, this is the best I could do.

Since yellow light turns it ruby, I thought I could hit it with some silver stain to bring yellow into the mix, and have a piece that was actually red. I fired a test chip and when I pulled it out of the kiln, I was shocked to discover I couldn't see where the stain was. I was a bit disheartened and considered plans B (enamel) and C (piecing it together). Then I realized I couldn't tell where the stain was on my test chip because the entire piece had turned ruby.

This is actually the unfired piece from the picture above lit by a different lightbulb, but this is how the finished piece looks in all light as you can see in all the other pictures.

That was fascinating and a surprise!