Murrini cane in a kiln: Sandwiches, mokume gane cane, and compression

What’s the difference between murrini cane and pattern bar?*

Beats the heck outta me. I can only think of two differences:

• • • • Pattern bar typically isn’t stretched, layered, and/or compressed to reduce the pattern
      • The final pattern bar slices are (often) bigger than typical murrini

Both start with a huge pattern bar or rod. For murrini, you heat and stretch and compress the to make it much smaller, then chop it into slices. Since that’s usually done in a hotshop, people tend to think of this as a glassblowing product, a small-scale, stretched/distorted cane that can be used to make repeating patterns.

This post is about making small-scale, stretched/distorted cane that can be used to make repeating patterns…in a kiln. I’m still calling it 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 cool thing about murrini, though, is the detail: If you start big and streeeeeetch until the glass is very small, you’ll get an amazing amount of beautiful, tiny detail in each piece. Typically kilnformed pattern bar layers are too big, too crisp, and lack the tiny detail of murrini.

You CAN get tiny detail in the kiln, potmelt fashion, by suspending the glass over the mold and letting it drip in, compressing it with weights, or simply cutting, remelting, cutting remelting…but the other requirement here is consistent, repeatable patterns. Probably not gonna happen with the randomness of a potmelt.

Update: Of course, you CAN achieve some pretty remarkable results with a vitrograph (a small kiln that holds a potful of glass which gets very hot and streams out a hole in the bottom of the kiln) by stacking the glass in consistent patterns. Even then, though, the pattern tends to evolve throughout the cane, so getting the same pattern/color throughout can be difficult.

So this is the assignment: Make a series of pattern bars that:

1. Have small-scale detail
2. Can be sliced into individual”cane” that can be chopped into murrini
3. Allow each cane to have the same small, repeatable color and pattern
4. Doesn’t look like a simple sheet stack

Futzing around with traditional pattern bar techniques and adding point distortion, compression, and flow, on a small scale, is giving me what I want. I call them sandwich murrini. They’re a lot of fun, with almost infinite variation, and are 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?

It takes more equipment to do sandwiches than it does the jellyroll and rod mold murrini I’ve discussed. Most experienced kilnformers, though, either already have this stuff or know where to get it:

• 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.
• Superglue
• 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 get hooked on this process, save yourself some time by making or buying a mold box, an open box of vermiculite or ceramic to hold the pattern bar while it cooks. You’ll want to add at least one 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.)

The 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.

Probably the easiest to make is one of vermiculite, with stainless steel screws to hold it together. Bullseye’s box casting tutorial has really complete instructions.

You can also buy “saggars,” mullite or ceramic vessels that are meant to isolate a delicate ceramic piece during firing (or to reduce oxygen for certain types of ceramic firing). They’re typically not that expensive; I’ve also seen ceramic stepping stone molds that would serve (make sure it’s a “refractory” mold that will stand up to prolonged kiln temperatures, however).

Whatever you use, it must be prepared properly, or your glass will stick, crack, and make a hideous mess. Start by coating box and lid with a few layers of kilnwash, drying carefully between coats. Do all sides of the lid; if you don’t, and you place weights on top of the lid, you can sometimes wind up with a lid embedded in the glass.

Kilnwash is only the start, however; you’ll also want to line your box with fiber paper. I usually use 1/8 inch. Figure out how tall your glass stack will be (before firing), then cut a strip that’s about 6mm shorter (1/8 inch) to go around the entire circumference of the box. Place it in the box, letting the ends overlap slightly.

Now cut a bottom piece that’s between 18mm-25mm (3/8 to 1 inch) bigger than the inside of the box. Slide this piece down into the box, all the way to the bottom; it will lock the sides in place.

And yep, the glass will show a bump wherever there’s an overlap in the fiber paper. I don’t care, because I’m going to trim 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:

1. Unless you’re a much better carpenter than I, you’ll make your vermiculite box with straight sides (and if you buy one, it will have straight sides anyway). Sliding stuff out of a straight-sided mold is tough, so the fiber paper gives you a compressible margin and some wiggle room. Literally.
2. 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.
3. 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.)

Calculating yield

How many murrini can you get out of a single bar made in a box like this? It depends. (that irritating, irritating answer)

The number of chips, or slices, you get from those cane depends in part 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 also want to know how much surface area you can get out of one sandwich bar, and that obviously will depend on the diameter of the bar.

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. Let’s say the bar is 2 inches (about 50mm) thick, and you want 1×1 inch cane. You first cut the bar into seven 1×2 inch canes 11.25 inches long. Cut those in half, and you wind up with 14 1x1x11.25 inch (25x25x285mm) canes, a total of 157.36 inches (3,996mm) of 1×1 cane.

More math. You choose to slice your cane with a tile saw, which can cut 3mm slices easily but leaves a 2mm kerf. Each slice, therefore, will take 5mm of your precious cane. So…

285mm cane / 5mm = 57 slices X 14 canes = 798 slices 1″ x 1″ each

Each 1×1 slice covers 1 square inch (obviously), and you have 798 of them, so you have 798 square inches of cane coverage at 3mm thick. That’s roughly 28″ x 28″ square inches. Of course, that 2mm saw kerf ate into your supply of slices. What if you chopped the cane apart, with a cane chopper of some kind? The slices won’t be as regular as with a tile saw if you’re cracking it apart, but you’ll have more slices:

285/mm cane / 3mm = 95 slices per cane X 14 canes = 1,330 slices 1″ x 1″ each

That just increased your yield by 40 percent or so, although you’ll lose some to cracking or irregular slices, especially with bigger canes. By chopping instead of sawing, your 28×28 surface area coverage just jumped to 36×36. Or thereabouts. THIS is why people chop cane instead of saw it apart, so if you’re getting into this stuff, it pays to learn how to chop.

Simple sandwiches

The simple sandwich is exactly that: A sandwich of different layers and types of glass, laid in your moldbox and possibly compressed during firing. The layers flow and spread into mostly even patterns. The messy-looking stack shown produced the aqua, yellow, and black murrini in the adjacent photo.

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, build in the box. Construct your layers all the way out to the edges of the box and pack as tightly as you can. The object of the game here is to give the glass as little room to expand, flow, and distort as possible.

If you want more painterly, watercolor-like layers, leave the last inch or two around the perimeter of the box empty, to give the glass room to flow to the edges of its dam/box. Without compression, the layers will be thickest in the middle, thinning out to the edges.

You can control the thickness of the layers (and the amount of flow) with temperature and time: The longer it’s in the kiln at flow-worthy temperatures (say, 1450F or above), the more distorted and flowy it will be inside, and the more even thickness you’ll achieve.

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 (without a box) or all over the box, 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.

BTW…

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 is NOT strong enough to lift by the edges, but it’s enough to keep stuff from falling out of the stack, and it cancels that pyramid powder effect. It’s also MUCH easier to transport the assembly to the kiln if you didn’t assemble in the box.

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 a more 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:

• 

  Anatomy of a glass sandwich

  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.
• 

  Dark colors layered together (here I’m using Gold Purple Opal, Cobalt Blue, and True Blue, with other lighter glasses for contrast) can make it difficult to distinguish your murrini layers unless you cut very very thin.

  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.

This stack was about 1.25 inches (30mm) thick before firing. After firing and spreading, it was 5/8 inches (16mm) thick.

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.

Compression bars
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:

1. 300 dph to 1100F, no hold
2. 50 dph to 1240F, hold 45 minutes
3. 50 dph to 1485F, hold 30 minutes
4. AFAP to 900, hold 4 hours
5. 30dph to 800, no hold
6. 50dph to 700, no hold
7. 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.

For this bar I added distortions with fiber paper as well as glass components to produce lots of ridges and bends. I sandwich and fuse the halves together, then do the same thing again, under weight, for a mokume gane effect.

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.

 

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*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.