sandwichsimplemurrini2What’s the difference between a murrini cane and a pattern bar?*

Beats me; I’ve only found two:

  1. Pattern bar typically isn’t stretched or compressed to reduce the pattern (and incidentally increase the number of murrini)
  2. 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.

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.

Here’s the rest of the series:

sandwichBWmurriniThe 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.

sandwichloaves1Equipment required. 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

And also for part II of this post:

  • Clay tools such as combs and ribs
  • Cheap earthen clay
  • Rubber suction cup (for lifting and dropping glass in tight spaces)
  • Refractory investment (50/50 plaster/silica mold mix works fine)

sandwichboxIf 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.

I lucked out and found a couple of old “saggar” boxes, used in ceramics for isolating smaller work in a big kiln, for $5 each at an artists’ moving sale. I made lids for mine by cutting up a broken kilnshelf about a half-inch smaller than the inside dimensions of the box, and chamfering the corners.

sandwichboxfiberHowever 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:

  1. 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.
  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? 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. If you don’t have a chopper, you can improvise one with a set of mosaic nippers, a cardboard box, and a 1-pound sledgehammer. (I talk about this in the jellyrolls post of this series)

Simple sandwiches

sandwichcrosssectionThe 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:

sandwich-firedsamples1To 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 worksurface 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.

Pressing your layers of glass (whether frit or sheet glass) all the way out to the edges of your box keeps the glass from moving, and makes your layers as flat and even as possible.

Pressing your layers of glass (whether frit or sheet glass) all the way out to the edges of your box keeps the glass from moving, and makes your layers as flat and even as possible.

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.

A stack can become a powder pyramid if you're not careful

A stack can become a powder pyramid if you’re not careful

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.

Glass boxes

Here’s an easier way: If you want flatter, more even frit layers, just glue 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.

sandwichglassboxTo 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.)

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).

sandwichglassboxinbox

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. (I’ve done that, it’s not pretty) 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).

But if you push the box up against one wall or corner, the glass will tend to flow out only in the opposite direction, so that the glass will appear to pile up on the side next to the box and become much thinner on the opposite.

Choosing glass

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.

slicesTo cut a bunch of stringer to length, lay them on your worksurface 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, so that it appears to be three or more layers.
  • 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–each particle seems outlined.
  • Stringers and rods can give you transparent windows or accent dots; if you line them up without spaces in between they give a scalloped effect.

The more the glass moves, the more your shapes will distort:

  • sandwich-crosssectionRods and stringer become more oval and will be carried along 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 unless the glass is really moving.
  • Sheet glass will distort to follow the contour of any accessory glasses underneath (as above, over the thick stringer), while powder and frit layers adjust so that the accessory glass penetrates the layer instead of distorting it.
  • Sheet glass layers definitely thin if the stack spreads, but they’ll do it most at the edges and almost never in the middle unless the glass becomes very “juicy.”
  • 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)
  • Powder layers will reduce by at least one-half thickness and blend together to make really interesting graduated patterns.

Color and transparency choices are just as important as the form of glass you’re using:

  • A lot of darker glass colors don’t work well in small murrini unless they’re well-separated with very pale colors or white, or they’re cut very thin. Dark colors can quickly overwhelm the design and make it ALL seem dark (above).
  • 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.
  • 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.
  • 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.
  • sandwichpendantTry 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.
  • 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 (as at right).
  • 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.

sandwichrecordboxI 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.

sandwichrecordThis 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.

sandwichmoldboxliddedCompression bars
You can stop right there, if you want, and you’ll have perfectly lovely miniature pattern bar for murrini cane. If you apply some compression to your pattern bar stack during the firing process, the layers will stretch and thin, and your bar will expand.

To do that, place another piece of fiber, cut to fit the ENTIRE space within the dams or the mold box (not just the area over the stack–remember you want this to spread out), on top of the stack. Top that with the saggar lid (or a prepared kilnshelf.

The kilnshelf topper by itself compresses the glass layers reasonably well. However, it’s still not much weight; even if you get the glass pretty melty, the layers will mostly thin toward the edges and stay thicker in the middle. If you want more compression, top the kilnshelf with additional weight.

sandwichweightsdamIn the picture below, 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 depending on how much flow you’ll want from the glass. I use a conservative bubble squeeze schedule since stacks are VERY prone to trapping air.

  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. What also varies is the process temperature–this schedule gives a complete, solid fuse but the glass doesn’t move much, even when compressed.

If you want more movement, especially when you distort and stretch the layers, increase the processing time–I generally hold at least 45 minutes at the top temp, but often go to 2 hours or more.

Once the stack is out of the kiln, you must figure out a cutting schedule to turn it into cane. I usually trim each rounded edge back to the original size of the stack–the trimmings make beautiful slices themselves, albeit a different pattern than the center of the stack. (I often use these for small sculptures or pendants).

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.

sandwichbwloavesThe second firing will creates tiny, subtle layers and some really beautiful patterns.

I take a couple of slices in each direction for reference, look for natural “breaks” that would make a good cane, then slice up the cane. The canes should be lightly sandblasted or hand-ground to reduce the chance of devit and pits, chips or saw marks.

Once you have a “cane,” you can either saw or chop it into individual murrini. Sawing’s NOT fast, unfortunately, but if you stack multiple cane and tape them together, you can slice them all at once.

Chopping is much faster, and produces a cleaner face without sawmarks. I much prefer it.

sandwichBWbuttonsThe 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…what if I WANT distortion? That’s a whole ‘nother topic…

—————–

*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 patternbar tutorial.