Making a Granulation Sieve
© 2008, Brian Meek
V1.5, All Rights Reserved
Granulation is a process by which large numbers of small silver or gold granules (balls) are fused onto the surface of a jewelry piece. There are several different processes to fuse the granules into place, but one characteristic they all share is a need for granules of similar size. It is possible to buy gold and silver granules, but they are absurdly expensive. The alternative is to make one's own granules. There are various tricks that will yield granules of approximately the same size, but for fine work, it is helpful to have the granules accurately sorted by size. Spending an afternoon with a microscope and a pair of tweezers is certainly one way to do this, but a more efficient method is to make a sieve.
A sieve is simply a cup or bowl with equally sized holes punched in the bottom. The holes are all of a given size, and will therefore allow items of smaller size to pass through. Imagine a dixie cup with the bottom replaced with window screen. That's a basic sieve. One sieve isn't much good, as it only sorts one size of item. A much more useful thing is a sieve stack: a stack of sieves with progressively finer holes, and edges that mate up with each other. Rough material to be sorted is dumped into the uppermost sieve (with the largest holes) and the whole stack is shaken a few times. This will cause the smaller particles to fall through the holes until they sort themselves out by size. Each progressively finer set of holes will retain those particles that are larger than its hole size, while permitting smaller particles to pass through. As applied to granulation, sieve stacks have one more advantage: storage. I have one sieve stack devoted to granulation, with various sizes of wire mesh screens. I just leave the stack screwed together, and then unscrew whichever compartment contains whatever size of granule I need. When finished, I simply screw the stack back together and store it (and all the various sizes of granules) until next time.
Granulation Sieve. Closed and open.
To make a sieve stack
The first thing to do is to find some of those round plastic containers that screw together. I've seen them in craft supply stores, as well as fishing stores. They come in two sizes: small, which is 2" diameter, and large, which is 2.5" in diameter. For granulation, the small size is plenty big enough. Get two sets. (In Santa Barbara, the K-Mart out at Storke has them in the fishing supply isle, and Michael's crafts has 1" diameter stacks in with the beading supplies. )
The next step is to make holes in the bottoms of most of the cups, to sort out your granules. There are two ways to approach this, and I'm of two minds about which is the easier.
When I was in grad-school, I needed to make a sieve stack to sort out chemical particles for something I was working on. As I needed particle sorting down into sizes measured in microns, I had no choice but to buy sieve cloth and make real sieves. The size of the cloth was such that the scrap left over fit the small 2" screw containers perfectly. Since I had the scrap sieve cloth anyway, I used that leftover cloth (which looks like fine window screen) to make my granulation sieve. The drawback to that is that sieve cloth is expensive. Expect to pay about $35 for enough to do a stack. The cloth comes in 3x6 sheets, so there's plenty for two or three people to team up on an order. That'll get the price down, but it's still expensive. (Details at the end.) This is what I did, because of what I had. It may not be the simplest road home.
The other process is simply to use progressively smaller drill bits to drill holes into the bottoms of the containers. The drawback is that it takes a long time to drill all those holes, and you have to have the right drill bits. The other issue is that the thickness of the plastic bottom relative to the diameter of the holes makes it easy for the holes to get plugged. A better idea would be to use thin (26 ga) brass sheet to make a bunch of disks to fit the insides of the screw containers, and then drill the holes in the brass. Make sure you lay out a real pattern, and follow it. Don't just drill holes randomly. There is some risk of causing one hole to merge with another. This oversize hole will ruin that sieve disk. Get as many holes into the disk as you can, evenly. It'll sort faster if there are more holes. Use light pressure, and sharp bits. Do not bend or deform the brass sheet. A good idea is to drill into a sacrificial piece of plexiglass for each disk, to insure that each hole has support while being drilled. Once the holes are drilled in the brass, treat it like the sieve cloth in the method below.
To make a sieve stack with sieve cloth:
Once you have either your sieve cloth, or drilled brass disks, the next step is to cut out the bottoms of the screw containers. Cut out the centers of the bottoms of all but one of the containers. (or however many you need for all of your sieve grades. Make sure you leave one intact to be the bottom of the stack.) The goal is to cut away the bottom so that you end up with an internal lip about .25" wide. Many of these containers have a round mark on the bottoms that will give you something very similar if you follow it. A jeweler's saw with a coarse blade will cut the plastic like butter. Once the centers are cut away, use a half round file held at 45° to the hole to file off the crumbs from the sawing. Pinch a bit of 320 grit sandpaper between your thumb and first finger, and rub the top and bottom of the lip briefly to smooth out the cut edge, and remove any plastic crumbs.
Once the holes are cut and smoothed, the next step is to cut the sieve cloth or brass to fit the inside of the screw containers. Place the sieve cloth or brass down into the container, so that it rests on top of the little lip that remains from the bottom of the container. Use silicone caulk to glue the sieve material into place. Make sure to leave a wedge of caulk up against the edge of the container. The goal is to create a 45° slope with the caulk, so that any balls that land there slide down onto the sieve, instead of getting stuck on the caulk.
Set the caulked containers aside and let them dry. Assemble them in order, and mark both the order and the hole size, and you're done. Time to start sieving. Dump the granules into the top container, screw on the lid, shake the stack a few times, and you should have nicely sorted granules, all ready to go.
Sieve cloth, hole size and suppliers.
The sieve cloth I purchased came from Small Parts Inc. I was working with granules down to .010" in diameter, so my sieve has many graduations. It starts at .0787", and works down to .0098" in 8 steps.
Mine has a mix of stainless and nylon screens, but that was a product of using scrap from another project. The page for the nylon screens is here. I used sizes .0787", .0394", .0350", .0223", .0197", .0150", .0138", .0098". They have a greater range of sizes available now than they did then. If I were doing it over again, I would use all of the new sizes available in between my current sizes in the upper range (from .0787" down to .0223" ) as those are the size granules you're most likely to want. Granules under .020" inch are a major pain to use, and even I rarely use granules smaller than .015".