Beats the heck outta me. So far I’ve only found two:
- Pattern bar typically isn’t stretched, layered, and/or compressed to reduce the pattern
- The final slices are generally bigger than typical murrini
In fact, for many types of murrini you start with a huge pattern bar, then heat and stretch and compress it until it becomes murrini cane.
So I’m making small-scale, stretched/distorted pattern bars with multiple repeating patterns…as far as I’m concerned, they’re murrini cane. Square murrini cane, produced in a kiln.
BTW, this is part of a series that I *still* haven’t finished–never knew there were so many ways to make murrini in a kiln. This post was SUPPOSED to be three parts but I can’t figure out where to split it, so apologies for the VERY long read.
The problem with simply stacking sheet glass to make a pattern bar for murrini, though, is that the layers tend to be too big, too crisp, and lack the tiny detail common to murrini.
You can go the other direction with a potmelt-style pattern bar by suspending the glass over the mold and letting it drip in…but you’re less likely to achieve consistent, repeatable patterns needed for murrini.
How can I make small pattern bar in the kiln with beautiful detail and a wide variety of layer thicknesses and shapes so it’s not a dead giveaway for a sheet stack…while keeping a tiny, murrini-like pattern?
If I fudge with traditional pattern bar techniques just a bit I can get pretty close. Technically, I’m making small-scale, compression-distorted, variable point-loaded, multi-fired mini-pattern bars.
But it’s easier to just call them sandwich murrini. They can be precise and rigidly controlled or gloriously wild. The variations are almost infinite, and frequently gorgeous.
I don’t claim to have invented this, it’s centuries-old, certainly practiced by many, many outstanding kilnforming artists. This is just my exploration–your mileage may vary.
What do I need to make sandwich murrini?
Kiln furniture (lots of it, in various sizes and shapes)
- Small or broken kilnshelves to use as stack weights
- Stainless steel shapes (rods, balls, anything heavy)
- Frit applicators (sifters, spoons, scoopers, brushes)
- Tweezers, ruler, mosaic cutters, glass cutter, scrap paper to catch unused frit, markers, etc.
- 1/8 inch fiber paper (LOTS of it)
- Wet tile saw with a GOOD, thin diamond blade
- Wet flat lap, belt or drum sander (or diamond hand pads and mondo patience)
- (Optional) Sandblaster
- Clay tools such as combs and ribs
- Cheap earthen clay
- Rubber suction cup (for lifting and dropping glass in tight spaces)
- If you want to make a one-time mold box, refractory investment (50/50 plaster/silica mold mix works fine)
If you plan to make sandwich murrini regularly, save time by making or buying a mold box. This is an open box of vermiculite or ceramic, plus a piece of old mullite kilnshelf cut to fit inside the lined box.
The shelf acts both as a weight to compress the glass and a lid to evenly distribute whatever additional weight you want to stack on top. (Even if you don’t make a box and use kiln furniture instead, you’ll still need the kilnshelf to compress the stack.)
A mold box helps develop consistent sandwich stacks, and really speeds up setup time in the kiln, since you don’t have to build individual, fiber paper-lined dams around each stack. Bullseye’s box casting tutorial can show you how to make a vermiculite mold box.
However you obtain a box, though, you’ll want to prepare it. First, coat the box AND lid with kilnwash (do all sides of the lid), then let it dry.
Now line the box with fiber paper; I usually use 1/8 inch. I cut the side strips first, about 6mm (1/8 inch) shorter than the anticipated height of my stack.
Then I cut a bottom piece that’s perhaps 18mm to a full inch larger than the inside of the box. Slide the bottom piece down the sides of the fiber paper; it will lock and hold the sides in place.
Yes, I’ll get a bump where the fiber paper bottom overlaps the sides. I don’t care, because I’ll be trimming off the edges, anyway.
If I’m careful, I can get two or three firings with the same fiber paper in place. No, you can’t skip the fiber paper step, for three reasons:
- Unless you’re a much better carpenter than I, you’ll make your box with straight sides (and if you buy one, it will have straight sides). Sliding stuff out of a straight-sided mold is tough, so the fiber paper gives you a compressible margin and some wiggle room. Literally.
- Frit shoved up against a solid barrier prevents air from escaping; frit shoved up against fiber paper can push trapped air out through air pockets, helping to reduce bubbles and pits.
- Four hours at processing temps can give your stack enough time to really press the air out and give you a denser glass pack. However, it also really tests the kilnwash. Fiber paper provides added insurance. (I wouldn’t try it without fiber paper, actually; I tend to think of the kilnwash as merely the backup.)
How many murrini can you get out of a single bar made in a box like this? Here comes that irritating, irritating answer: It depends.
The number of chips, or slices, you get from those cane are going to depend on how you choose to cut them. If you use a diamond tile saw, the precision and thickness of your saw blade play a huge role. If you’re planning to use your murrini to cover a flat surface, like tile, you’ll want to know how much surface area you can get out of one sandwich bar.
You can calculate it this way:
An 8 x 12 inch mold box yields about 7.25 x 11.25 inches of usable sandwich bar if you’re careful about trimming off the edges. If you cut it into 1-inch canes, that’s seven 11.25-inch-long canes, or 78.75 inches of 1-inch cane. Let’s say the bar is 2 inches thick, and your tile saw can cut 3mm slices easily, with a 2mm kerf. It’s therefore going to be grabbing 5mm with every slice. So…
285.75mm cane / 5mm = 57 slices X 7 canes = 399 slices 1″ x 2″ each
area coverage = 1″ X 2″ x 399 = 798″ squared, or about 5.5 square feet (roughly 28″ x 28″)
wasted glass from saw kerf 285.75mm – (57 X 3mm) = 114mm or about 40%
A tile saw gives you the straightest, most regular and even murrini slices. However, diamond blades don’t cut the glass, they grind it away. The amount they grind away–the width of the saw blade–is lost material from your precious glass bar.
Any chipout or cracking from a misaligned or inadequately cooled and lubricated blade will also waste your glass. (For more on this, see my tutorial on using a glass saw)
If your bar isn’t too thick, or you have a cane chopper, you can chop the cane. The slices are more irregularly shaped because you’re cracking the glass apart instead of grinding it apart, but you lose much less.
The simple sandwich is exactly that–a sandwich of different layers and types of glass, laid in your moldbox and compressed (or not) during firing. The layers flow and spread into mostly even patterns. These murrini were produced by the above stack:
To make a simple stack, cut a bottom sheet of glass and place it on your turntable.
If you don’t have a turntable, use on upside-down paper cups. It’s essential NOT to have the glass lying flat on the work surface because it’ll be almost impossible to lift it without disrupting your stack and have glass flying all over your work table.
I begin and end my stack with sheet glass; it makes handling easier and provides pattern continuity. The size of the bottom sheet will depend on how many murrini you want, the size of your box (if you’re using one), and how much you want the stack to thin and move.
If you want the layers to stay relatively straight and even, construct your layers all the way out to the edges of the box and pack as tightly as you can. You want to give the glass as little room as possible to move.
If you want more painterly, watercolor-like layers that ebb and flow, give your stack room to flow to the edges of its dam/box. I generally give my stacks at least an inch of “spreading room” on each side. The layers will be thickest in the middle, and thin out at the edges.
If you’re using frit and powder, remember that it will fall off at the edges of the stack, creating a pyramid effect with the thickest area in the center and almost nothing on the edges.
It will also be very difficult to move without dropping frit and other pieces of glass all over the place, and driving yourself nuts. One way to get around this, if you’re using relatively large pieces of glass, rods, or stringer, is to simply superglue everything in place.
Use as little glue as possible, but since you’re cutting cross-sections, any glue residue probably won’t show. You may, however, get more bubbles, and it’s a pain in the neck to apply all that glue.
According to the web, murrina refers to a slice of complex cane, murrino means an insert of colored glass. Murrhine describes a patterned, multicolored stone like agate, which in ancient times was used to make vessels. Murrine can either be a mosaic of small, patterned pieces of glass or cross-sections of a circular or square rod of glass with extremely intricate patterns. Murrini is the way non-Italians spell murrine, apparently, because it reminds them that Italians have this weird way of semi-pronouncing the E at the end of a word.
Murrini. Murrina. Murrhine. Murrine. Murrino.Whatever.
Glass boxes make your life easier
If you want flatter, more even frit layers, put your stack in a glass box. Simply superglue glass sides to your bottom sheet to make a 5-sided box. The box won’t be all that strong, but it will be enough to hold in your glass and cancel that pyramid powder effect. And it will make it MUCH easier to transport the assembly to the kiln.
To make one, cut a strip of glass about up to about an inch taller than you plan to build your stack (before firing. Cut the strip up into box sides to fit the edges of your bottom sheet. Superglue the strips to the bottom sheet.
Don’t worry about neatening things up too much; your box doesn’t have to be perfect. And since you’ll be cutting the edges off your fired stack, you can use any color of scrap glass for the box sides.
CAUTION: The box will be fragile–DO NOT try to lift it by the sides or you will be covered in ex-stack glass. Promise.
In a box, frit can layer all the way to the edges without falling off, producing an even line. Remember, however, that any sheet glass you plan to use in the stack must be cut to fit INSIDE the glass-sided box (i.e., cut it smaller than the bottom sheet).
You can set your glass box in the mold box before filling; the combination provides extra insurance that you won’t drop the assembly on the way to the kiln. Where you put the box inside the mold makes a difference in how the layers will settle; if you center the box in the mold you’ll get an even flow more or less on all sides (assuming your kiln heats more or less evenly).
Push the box up against one wall or corner and the glass will flow out to the opposite side. The part of the stack nearest the wall (and on bottom) will move least, while the side with the most distance to travel will flow forward, trying to get into a full horizontal. It will also get thinner, the farther out you go.
Choosing glass for stacks
If you use a softer glass for top and bottom sheets (such as black, which flows well at lower temperatures), the murrinis will blend together almost seamlessly when laid up mosaic-style, especially when using same-colored frit to fill in gaps.
The edges will also be more forgiving of chips made by a tile saw, and more likely to heal without a trace in the firepolish.
Harder glass colors, such as white, aren’t nearly as forgiving–you’ll need to (sometimes greatly) increase the amount of heatwork to get the same effect as with black.
Once you’ve established your base and top glasses, you can layer just about any form of compatible glass in between, as long as it will fit onto the stack. Ideally, you want accessory glasses like rod and stringer to stay put (use superglue or embed them in frit), and they should be as uniform in length as possible so your pattern stays consistent.
To cut a bunch of stringer to length, lay them on your work surface and square up one end, then draw a line across the break point. Run your glass cutter across that line a couple of times, then pick up the stringer and wiggle it gently, back and forth, at the break line–the stringers should easily separate at the right spot.
Remember that you’ll be cutting your fired sandwich into bars, and then into individual murrini, and you want your pattern to translate well as a single, repeatable murrini slice.
For example, if you want a yellow circle in the center of each murrini, lay rods of yellow glass all the way across the stack, between the cutting lines to make cane, and perpendicular to the cuts made for murrini slices:
The form of glass you use greatly influences the design:
- Sheet glass gives you the biggest, crispest lines, and may turn a different color or transparency at the edges, giving you thin outlining layers for free.
- Powder gives a more flowing line, with softer, more blended edges.
- Larger frit particles can add beautiful detail, especially if they’re next to a layer in a strong contrasting color in powder or reactive glass. Each particle will seem to have an outline. If you use transparent glass, you’ll get a crystalline, randomly shaped effect.
- Stringers and rods can give transparent windows or accent dots; if you line them up without spaces in between they produce scalloped shapes.
The more the glass moves, the more your shapes will distort:
Rods and stringer become more horizontally oval and will be carried forward as the glass moves.
- Narrow strips and noodles will tend to move with the sheet instead of distort, and they don’t seem to thin much unless the glass is really moving or compressed.
- Sheet glass distorts and follows the contour of any accessory glasses underneath (as around a thick stringer).
- Sheet will definitely thin as the stack spreads and flows, more at the edges than in the middle of the stack. In fact, unless the glass is very hot and flowing (I call that “juicy”), you’ll probably get almost no mid-stack distortion.
- Powder layers will reduce by at least one-half thickness and frequently blend together to make really interesting graduated patterns.
- Powder and frit layers are more likely to “absorb” accessory glass, letting the pieces settle in so that they penetrate those layers instead of distorting them.
- Air bubbles tend to get stuck under sheet glass; if they rise through the glass you’ll get a break in the line (which can be fun); otherwise, you’ll get a hole. Don’t despair–murrini holes can be filled with contrasting glass, refired and turned into some very cool pendants. (NOTHING is wasted in one of these stacks)
- Air bubbles in power/frit will change depending on process time in the kiln; the more time, the more the air flows together to make bigger bubble cavities.
Color and transparency choices are just as important as the form of glass you’re using:
- Go for contrast. It’s best to alternate color saturations for contrasting layers, i.e., dark, light, dark, light–murrini tend to concentrate saturation. Dark colors can quickly overwhelm the design and make it ALL seem dark (above).
- Mix opacities. A combination of transparent and opal glasses makes for the most interesting murrini; if you want to use all transparent, use strongly contrasting colors or the murrini will look dull.
Avoid coatings. With maybe two exceptions (I’ll talk about them in the next post), sparkly surface coatings like irid and dichroic are a menace in murrini; they don’t add much sparkle because they’re generally viewed on edge, but if you do a full fuse with irid/dichro-containing murrini the coatings tend to float to the top of your project and look awful.
- Monitor opalines. You CAN use Bullseye’s new opaline frit and sheet to great effect in these murrini, but remember that it’s very sensitive to heatwork and use the coolest, shortest schedule possible. If it’s already opalized in the murrini, opaline will likely go nearly opaque in the finished piece.
- No more than 4-5 colors. Try to limit your color palette–you’re working on a very small scale and slices can quickly become too busy if you load up on a variety of color.
- Use a feature color (or two). Lighter, monochromatic palettes work well as the base coloring, especially if you use thin layers of darker values to separate them. My favorite murrini usually start with a pale, monochromatic palette, then add one or two accents of strong contrast/complementary color.
- One option: Track the layers you’ve added while you build the stack, and when you get to the center simply reverse the order to fill up the rest of the way.
I assemble ALL the glass in the palette on one side of mold box before I start, then use a Sharpie marker to jot down my build order directly on my (glass) table top as I’m working. Later I can transfer it to firing records and clean off the table.
How high do you stack? Different forms compact differently, especially when compressed, so it can be hard to predict the thickness. Usually, though, I figure the stack will lose about 50 percent of its height once fired.
In this example, I’ve stacked five thin (1.6mm) sheets, so I know I’m starting with at least 8mm of glass. Powder can reduce by about half; fine frit by about a third, and the glass footprint will expand by about two inches in each direction if you choose not to dam.
To fire, either set up conventional dams and fiber paper around the stack on a kilnshelf, or put the mold box in the kiln, up on stilts (so that the heat can also flow under the box.
Stop right there, and you’ll have perfectly lovely miniature pattern bar for murrini cane. However, if you want to kick things up a notch, apply some compression to your pattern bar stack during the firing process. The layers will stretch and thin, and your bar will expand and distort.
It’s easy to do with a mold box. Simply cut another piece of fiber to fit the interior of the mold box (not just the stack footprint). Place it on your stack, then top with another piece of kilnshelf.
The kilnshelf topper will compress the stack layers fairly well. However, if you get the glass pretty melty with a long process time, you’ll get even more compression. The layers will still mostly thin toward the edges and stay thicker in the middle, but the shelf will press them down a bit, encouraging them to thin out.
To really smear those layers into ultimate thinness, however, you’ll need to add weights to the kilnshelf. This can be firebrick, a bowl of stainless steel shot, more kilnshelf pieces…just about anything that will stand up to the heat without spalling (oxidising and flaking dark muck all over your glass).
Here, I’ve used heavy stainless steel tubing and kilnbrick inside dams, probably about 8-9 pounds, to really force the glass down and out.
Firing schedules vary for compression depending on how much flow you’ll want from the glass. Remember that you’re adding insulation to your stack (instead of just firing on an open kilnshelf, so the glass will take longer to heat.
I process as if I were casting an intricate mold, to get the glass “juicy” enough to respond. I also use a (very) conservative bubble squeeze schedule since stacks are prone to trapping air. This schedule, in the pictured configuration, will give very even and slightly thinner layers in my kiln:
- 300 dph to 1100F, no hold
- 50 dph to 1240F, hold 45 minutes
- 50 dph to 1485F, hold 30 minutes
- AFAP to 900, hold 4 hours
- 30dph to 800, no hold
- 50dph to 700, no hold
- OFF (cool at kiln’s natural rate)
Obviously your schedule will vary according to your kiln’s capabilities and the thickness of the stack. If I want the layers to really thin out, I’ll increase the process time (Step #3 in this schedule) to two to four hours.
Once fired, you’ll need to revisit your cutting schedule. I SHOULD stick to the planned cutting pattern (as in the earlier diagram), but usually I’ll trim off the edges of the stack to start my cuts and notice something beautiful going on in the opposite direction. I’ll wind up cutting the stack in half, one where the cuts will stick to the plan, and the other half where I’ll be making diagonal cuts, side cuts, whatever, and getting some really gorgeous pieces.
BTW, those edge trimmings make incredibly beautiful slices so play around with them a bit. They’ll have a different pattern than the stack, usually wildly variable and somewhat akin to burl or crotchwood in woodworking. They can be ganged onto a clear sheet for incredibly intricate and organic patterns or stand on their own as small pendant/sculptures.
Double, triple, quadruple your bar
Typically, if I’m going for compression bars, I’ll cut the results of the first firing in half, stack those pieces atop one another, weight them down again, and refire with a long process time.
That second firing will creates tiny, subtle layers and some really beautiful patterns.
It’s hard to plan anything for cutting with those, so I’ll trim the sides, take a couple of slices in each direction for reference, and look for natural “breaks” that would make a good cane. Typically there will be at least two great directions to cut, so I’ll slice both ways.
Cut canes should be lightly sandblasted or hand-ground to reduce the chance of devit and pits, chips or saw marks, even up the edges, and just generally get ready for uniform slices.
Once you have a “cane,” you can either saw or chop it into individual murrini. Sawing’s NOT fast, unfortunately, but if you stack your canes in the original order and tape them together, you can slice them all at once.
Chopping is much faster, and produces a cleaner face without saw marks. If you’ve got a murrini chopper, great. If not, chopping cane by hand quickly wears out your wrists and can cause repetitive strain injury if you do too much of it. I build a makeshift chopping platform instead.
To do it, buy a cheap pair of mosaic nippers, about 6 inches of an old 2×4/block of wood about as thick as your choppers’ head, a box, and a 2-3 pound sledgehammer. Cut a groove into the wood so the top of the bottom half of your nippers are level with the 2×4 surface, and set the nippers into it. Place the whole thing toward the back of your box.
Now slide the cane into the business end of the nippers (you should be able to slide it along the 2×4), position the blades over the right spot, and –WHACK– the top of the cutter head with your hammer. The hammer gives more power to your cut without hurting your hand, and the box keeps murrini from flying all over your studio.
Note that I said “buy a cheap pair of mosaic nippers.” I wouldn’t recommend doing this to your nice tools.
The simple sandwich gives me hundreds of murrini in a consistent color palette and–if I’m careful with my stacking–consistent pattern. Usually there’s just enough variation to keep the murrini from appearing to be mechanically produced, but the layers stay fairly straight and true, without distortion.
Mokume gane. Sorta.
But…what if I WANT distortion? What if I propped up layers of sheet glass and frit with glass “chads” so that the glass softened and melted into distorted concentric lines? I kinda sorta get the glassist’s version of mokume gane, AKA pattern-welded metal, my very favorite-on-the-planet metalworking technique.
Typically I do distortions with just glass, deliberately adding murrini chunks, rod, pieces of billet, whatever, to my stacks to ensure that the smooth glass layers will distort while softening and compacting. But I can also add non-glass, such as stainless steel balls to the bottom of the stack, providing significant, immovable distortions for the glass.
CAUTION: I’ve learned from sad experience (and a bunch of glass-embedded steel marbles) to never let the glass flow COMPLETELY around the balls. I typically cut enough fiber paper/board so that only half of the marble is doing the embedding, making it much easier to remove.
Which reminds me: You can also do this with stacks of fiberpaper.
I can then take the distorted stacks, cut them into half or quarters, and stack them again.
They’re uneven, of course, so I fill
the gaps with frit and glass scrap, providing additional distortion. I typically wrap that concoction with scotch “magic” disappearing tape (it tends to fully vaporize in the kiln where other tapes won’t) so it holds together.
I place that in my saggar/mold box, in the center, ensuring I’ve got at least a couple of inches of space between the edge of the box and the stack. I weight that down, as described before, and give it a long, slow fire.
What comes out is a flattened pattern bar with very thin layers and obvious compression. If you slant the moldbox by placing kiln furniture under one side, you’ll get even more flow and distortion, and you can do the process all over again. Each time, the layers will thin out and become more wispy and watercolor-like.
Or you can opt to leave the bar with distortions intact. Sometimes the distorted bar shape is beautiful all by itself.
*Pattern bar is a big ingot of different colored glasses, fused into a solid mass. You slice across a bar to make thinnish pieces of patterned glass that can be laid end to end to make even bigger pattern sheets of glass.
If you want to see some exceptional examples of pattern bar making, check out the early work of Rudi Gritsch (who, incidentally, has a most informative Corning video on the subject) or Lisa Allen, whose work always makes me happy. Or Dr. Steve Immerman, whose excellent website includes a great pattern bar tutorial.