Murrini cane in a kiln: The rod mold

FINALLY I’m back in the studio after a seven-month hiatus. I figured I’d start with something easy: Making components for bigger sculptures. Then it turned into this bigger thing, i.e., surveying methods for making murrini cane in a kiln. The whole thing is ‘way too long to publish in a single post, so I’ll break this up into sections. Sorry about that.

I’ve got some ideas for cast, figurative sculptures and vessels that incorporate murrini, bronze and other things. I’m not entirely sure where this is going, i.e., I want to play around a lot before I finalize the series design, so I need a LOT of murrini to experiment with…and I set out to make a bunch.

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:

• Murrini cane in a kiln: Sandwiches, Part I
• Murrini cane in a kiln: Jellyrolls
• Kilnformer’s murrini you can buy
• Kinda like peanuts

Anybody reading this probably already knows murrini well, but just to make sure we’re on the same page: Murrini are small, round “coins” of glass with colored patterns on the flat face that go all the way through to the other side. They’re one of the oldest methods of embellishing glass; most people regard them as primarily Venetian but murrini have been found in Mesopotamian digs dating back 3,500 years.

Murrini are generally sliced from cane, i.e., long cylinders of glass, and they can be amazingly detailed. They’re a great way to obtain repeating patterns across an expanse of glass, and people tend to pave surfaces with them, mosaic style. Millifiore paperweights are done this way.

They can also be used to make components that become bigger pieces. People who do portrait canes frequently make up eye, lip and nose cane, for example, and they’re often used to make letters or the artist’s signature. Kristien Berghs has a really nice demo showing how to make a cats-eye murrini on her blog–take a look.

The way you make them varies with the equipment you use. Glassblowers tend to work large, pull lots of cane, then chop it into the right sizes for whatever they’re working on, either as true murrini or simply short lengths of cane. Check out Lino Tagliapietra making cane in Corning’s wonderful series on the subject.

Torchworkers make cane by applying rods of glass to each other kinda like toothpaste on a brush. Like glassblowers, they build a kind of magnified version of the final cane in a short, fat cylinder. Then they apply a lot of heat and gently stretch the cylinder to its final, much thinner dimension.

Folk like Dinah Hulet and Loren Stump take this technique to extremes, making incredible, extremely detailed murrini that look more like paintings than stacked pieces of colored glass, pulled and reduced to miniatures. One of my favorite glassblowers, Stephen Rolfe Powell, exploits the wonderful layers of color in his murrini to make his enormous vessels. Giles Bettison builds murrini into kilnformed or rollup vessels that are almost calligraphic in detail.

I’ve made cane and murrini at the torch and in the hotshop, so when I started fusing glass one of the first things I did was try to make supply of murrini for the kiln. Almost immediately I discovered that a LOT of things change when you can’t directly place the glass into a torch…and there are lots and lots of different ways to get around that.

(Disclaimer: I’m NOT claiming to have invented any of this stuff. Nor am I an expert–these posts document what I’ve found and hopefully give you a starting point for your own explorations.)

Kiln murrini method #1: Vitrograph cane
The vitrograph method probably most closely approximates hotwork, since you can actually manipulate the hot glass. A vitrograph is simply a small kiln with a hole in the bottom, raised up on stilts.

You charge the kiln with a flowerpot full of glass, let it get really really juicy (1500-1700F), and then grab the cane as it flows out of the hole in the bottom. You can chop it up to make murrini; whether or not it is more than solid glass depends on how you’ve loaded glass into the flowerpot, how much heatwork has gone into the glass, and where you are in the pull.

Nathan Sandberg gave an ultracool demonstration of canemaking at the Bullseye Resource Center last fall (above). In the picture he’s using a vitrograph to make classic Italian cane, a la Lino Tagliapietra, and the ones I saw him pull are strikingly close to traditional latticino.

They would make intriguing murrini. And since cane length is really only limited by how you manage the glass before it hits the ground (and how much the flowerpot holds), you could get a LOT of similar murrini this way.

Update 3/1/16: Since I wrote this post, Nathan has left Bullseye–he’s a well-respected artist in his own right–and has turned vitrograph canemaking into a true artform.

He teaches classes in this, and I highly recommend taking one if you get a chance.

He’s also very sensitive about sharing his methods, so I won’t. But here’s a sample of something I did in one of his early classes.

However, since you can’t precisely group and control the glass(es) as they emerge from the hole, it’d be difficult if not impossible to build classic millifiore with this method. I played around with the idea of running multiple pots of different colors in a larger kiln, and combining the strings of glass on the way down to obtain some of the classic patterns, but I’d think it’d still be tough to make them consistently.

(Although if anyone can figure this out, Nathan probably is the guy)

But what if you don’t want to cut a hole in the bottom of your kiln? (Or buy a vitrograph?) Turns out there are LOTS of ways to make murrini directly in a kiln; some get you closer to torch-style murrini than others.

Murrini that are entirely kilnformed probably won’t give you the control over color and pattern that you get from manipulating the hot glass by hand, but that doesn’t mean they’re not beautiful and quite useful. Or that they don’t have their own strengths.

So, let’s start with the easiest:

Kiln method #2: Commercial murrini mold AKA “the rod pod”
Most kilnformed murrini cane is simply small-scaled pattern bar. You load glass into a mold, fully fuse the contents, then pull out the cane and chop it up. You can buy commercial molds to do this relatively inexpensively. Typically they’re called “rod pods,” “rod molds,” or “murrini-makers.” They are simply hollow squares with a series of long, rectangular channels to hold the glass.

Using a murrini-maker is almost a no-brainer, and you can stick just about anything in the mold channels: rod, stringer, frit, sheet, scrap. You can make rudimentary patterns just by being careful with the stack and, although something like a latticino (twisted) cane would be tough (I can’t think how you’d do it), simpler designs are fairly easy.

My favorite murrini mold canes start with a glass rod, then glue on or stack small stringers or noodles or tiny glass strips in successive layers until the channel is filled. They’ll melt into semi-regular patterns and you can obtain a lot of rather cool designs this way, especially if you mix transparent and opal glasses.

However, the channels in these molds are flat, so if the channel is much more than a quarter-inch wide the glass will pull out of round. What you’ll get more resembles a slice of French bread (an underleavened one at that). You can coldwork the rod back into round (or even square), but you’ll lose a fair amount of glass.

Also, it’s difficult to precisely control placement of your murrini elements in these molds. Usually, gravity pulls the glass down off the top, making the design lopsided. You also need to take volume into account, since using frit means you’re filling the mold with air between the particles, and when that air displaces (you hope), the glass will sink and spread. That will distort whatever shape you had in the mold channel.

If you learn to work within those limitations, though, this method can produce some beautiful stuff.

One caution: ALWAYS ALWAYS ALWAYS make sure there’s enough kilnwash in the channel, and that it’s not beginning to flake off. If you don’t, the cane can pick up the kilnwash to the point that you must coldwork it off. Ugh.

Making “eyeliner” cane
I’ve grown to love this one–it takes spray adhesive and a piece of scrap paper to make an eye-like cane with a detail line.

You can find the adhesive at hobby and fabric stores; my favorite is quilt basting spray but anything that stays tacky will work.

DO NOT spray this stuff near anything you care about, like a gorgeous cherry and ebony cabinet that until recently had a nice hand-rubbed finish, because it’s bloody near impossible to get the stuff off. If you have it, you could also use gold leaf sizing, which is less messy but not by much.

Start with a glass rod, transparent or opal, which contains one or more reactive elements–sulfur, silver, copper, lead, etc.), and two or more different glass powders. At least one of the powders should contain the opposing reactive chemical. Bullseye’s Reactive Ice/Cloud glasses would be very cool in this application.

If you’re not sure which colors are reactive, check out Bullseye’s reactives charts, or Lauri Levanto’s excellent charts for transparent and opaque Bullseye glasses, with a separate chart of the strongest reactions.

Generally (although not always), similar colors from other manufacturers will exhibit similar reactivities, but the intensity may differ.

(Hint: It’s helpful to copy the reactive info directly into the manufacturer’s catalog, right on the images of glass samples, then keep it handy in the studio for quick reference. I wish to blazes the manufacturers would do it for us.)

Cut the rod to length for your murrini mold, then spray it well (outside) with the adhesive. Now roll your sticky glass rod in any glass powder that will react with the rod (in this example, BE French Vanilla in Gold Purple).

Don’t worry about which color you use as long as it’s reactive–the color will pretty much vanish in the reaction.

The powder layer will lose at least half its thickness in firing, so you’ll need at least a double coating of powder for even a thin line. Just repeat the spray-and-coat process until the powder layer is about 2X the thickness of the line you want.

If you make the coating thick enough, you can sometimes get a thin line of the original color to show as well–it depends on the reactivity levels of the glasses.

Now dump a second color of powder into the murrini slot in the mold (in this example, I’m using orange opal), filling it until it’s about level with the top of the channel. Press a second (clean) rod down into the powder, stopping about a third of the way from the channel bottom, then wiggle it from side to side a bit to open up a trench.

Place your coated rod in the trench and top it with more powder, mounding it as high as you can get it on the rod. Compact it very firmly. The powder will almost certainly slide down the sides during firing, so if you don’t have enough on top your rod color will break through, like this:

But if the rod stays mostly centered, what you’ll wind up with is an “eye” cane with a fine, dark line separating the components, as below. The lengthier process time tends to slide and roll a bit, producing a more flattened cane that actually works very well when you’re “paving” these things, i.e., covering a surface with them.

You can make up to a dozen of these cane at the same time, depending on the mold, and they can be gorgeous. Mine tend to go well with my bandaids, a must-have when cutting up that many cane by hand.

Schedule-wise, a good full fuse usually works for these cane; the manufacturer’s schedule for a 3/8 inch piece of glass works pretty well. I’ve modified my schedules to give another 20-30 minutes at process temp than normal; this helps get the bubbles out and pulls the cane in a bit.

Here’s what I use (as usual: This is what works for ME; it may not work for YOU, so modify this schedule for your kiln’s firing characteristics):

1. 300F/150C dph to 1220, hold 30 (bubble squeeze)
2. 9999/AFAP to 1480F/804C, hold 30-40 minutes (process)
3. 9999/AFAP to 900F/482C, hold 1 hour (anneal soak)
4. 100F/38C to 700F/371C, no hold (slow cool)
5. OFF to room temp (fast cool)

I know that many rod pod users don’t worry about annealing because the cane will be chopped up anyway; they simply shut off the kiln for a natural cool-down. The cane is thin and symmetrical enough that it’s probably fine. However, I find that my cane cuts are straighter if I give it more of an anneal.

So far, my rod pod cane isn’t all that precise, and the rodpod shape is a dead giveaway to other kilnformers, one which produces lots of irregularly shaped, mangled-looking murrini if you’re not careful. It’s certainly not a match for murrini cane crafted at a torch.

If your outer layer is glass powder, the side inside the mold may have a sandpaper texture–the powder doesn’t always completely melt. That leaves you with murrini sides that are half-smooth, half-rough, and the rough side seems a bit more prone to scum/devit. It sometimes prevents a smooth, flat melt in a full fuse, so I like to do a fast coldwork on the rough parts.

So, what other methods work? Well, besides rod pods, I’ll look at jellyrolls, sandwiches, verticalities, and cast/shaped cane…as well as kilnforming cane you can buy. If you can think of any others, lemme know.