Introduction: Wood Mold for Glassblowing
This is an easy way to make a wood mold for blowing round glasses or vases or anything like that. The construction of this mold is inspired by a Czech or Russian (that's my guess) style mold that I came across while living in Denmark. This Instructable won't go too deep into the skill it takes to blow the pieces, but if you want to blow the pieces yourself, you might be able to find a glass studio in your area that offers classes. With practice, you can do it yourself. If not, a local glassblower will be able to make pieces from your mold, for a decent price.
Most traditional wooden molds are made from a solid piece of wood. The interior shape is hollowed out on a wood lathe, which requires a tremendous amount of skill and a special cutting jig that is mounted on the lathe. After the inside shape is removed, the wood is then split (if the shape has undercuts) and air holes are drilled through the walls.
For production glassblowing, the molds are usually made from graphite or metal. The advantage is that they will last a very very long time. The wood mold shown here is much much cheaper and easier to make, and you can still produce dozens upon dozens of pieces before it wears out. Plaster can also be used to make molds, but I find this very tedious and messy and prefer making these wooden molds instead.
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Step 1: Designing the Glass and Creating a Template
I wanted to make more of the drinking glasses that I'd designed a few years ago. I was down to one glass, blown by my mentor, the late Charlie Meaker, and became a bit concerned about breaking it. So I decided to make some backups that I could use everyday without worry.
The shape of my glass is pretty basic, just a low and wide drinking glass with a chamfered base. It doesn't have undercuts, so the mold-making is simple, and the glass can pull straight out the top after it's blown. Any shape with undercuts will require a split mold, which complicates things a little bit. A split mold will usually need an assistant to hold it shut, or some kind of catch to keep it together.
When designing a glass, it's most important to think about what you want to drink. In this case, I made a glass for all around use, from water, juice, beer, to wine or whiskey. It's blown fairly thin at the rim, but thicker at the base so it's still durable and has a good weight to it.
It's just as important to think about ergonomics. A glass that is too wide or too tall can be awkward to hold. Picking it up a glass should be easy, comfortable, and natural. A glass with a thick rim isn't very pleasant to drink from, and a very thin rim can feel too delicate for anything other than wine.
Glasses come in all shapes and sizes. Whiskey glasses are usually heavy, wide, and low. For juice, I like a tall and narrow glass. Shot glasses are always small and durable, and beer glasses can be so big they need handles. Wine glasses have a whole science attached to them, just look at the selection offered by a company like Riedel. For hot drinks, like coffee and tea, I prefer something ceramic. They usually have a handle, because the heat transfers to the walls and it can be too hot to hold.
This mold-making technique can also be used to create larger glass pieces, like vases. Think about what kind of flowers will be displayed. How long are the stems? How much water should it hold? It should be heavy at the base so it doesn't tip over easily.
Do some research, sketch out some shapes, and decide on a direction to take. It's fairly easy to create a few different shapes in a 3d modeling program, using the Revolve command. When you know what you want to make, create a 1:1 template.
The next step to making the mold is to draw out a full scale 2-dimensional template. I do this by laying out the shape on paper, connecting the dots, and then cutting it out with scissors. Use thicker card stock or 1/4" thick wood if you want to preserve the template.
Step 2: Cutting the Wood and Assembling
Use the template to mark out the profile onto the wood. Then cut the individual slats on a band saw or scroll saw as precisely as possible. The slats are then screwed to a base board, and the template is used to make sure the correct spacing is maintained.
Types of Wood:
It’s best to use solid wood if you can, especially fruit woods, like cherry, apple or pear. Plywood, Particle Board, and MDF will work, but the glue used in the wood can contain formaldehyde, which is toxic when burned (this is bad for the glassblower).
1/2" - 1" thick slats will work. Thinner slats will require closer spacing. 3/4" thick works just fine and is wide enough to easily screw into. The slats in the original mold that I saw were about 3/8" thick, but it wasn't attached to the base with hardware, instead it had more complex joinery. The slats used here are 1" wide because that's the wood I had, but I'd recommend sticking with 3/4".
Cut the Profile into the Slats:
Using the template, the profile is traced onto the wood with a pencil or scribe. The wood can be cut on a band saw, scroll saw, hand saw, laser cutter, or any other way you can think of to cut wood.
Each slat should be identical, but there is room for error, because as the slats burn in, the imperfections are evened out.
A plywood base board is needed. Just cut a square that is a few inches larger than the shape you want to blow. If the mold requires splitting, cut the base board so one half forms a V that fits into the negative-V of the other half.
In this form I've used two lengths of drywall screws (a shorter screw is used on the inside holes so they can't penetrate through the wood). Try to use screws that won't rust, because they will be in constant contact with water. Drywall screws aren't that strong but will work fine as long as the holes are pre-drilled.
Pre-Drill / Countersink:
To keep the wood from splitting, make sure all holes are pre-drilled. The screws in the bottom of the base board should be countersunk so the form is flat on the bottom.
If you need to be very precise with the shape of your finished piece, you have to take into consideration the fact that the wood will burn as the mold is blown into. Each slat will probably burn away 1/16" until it forms a layer of carbon. Because of this, your final piece might be 1/8" wider than planned. Adjust accordingly.
Attach Slats to Baseboard:
Use at least 2 screws per slat (so it won't rotate). I usually set each slat on the base board, make a pencil line around it, and then number the slats to they stay in the same place on the baseboard. First drill the holes in the baseboard, then drive the screws in until they're just above the surface (1/16"). Press the slat down into position, and this will mark the bottom of the slat so you know exactly where to pre-drill.
Drill Holes in Baseboard:
This is a step that I actually forgot, but a few holes should be drilled in the baseboard, directly under the piece. If you don't do this, what can happen is water will puddle up on the base and when the hot glass is dropped on top of it, it will expand into vapour and this can cause the bottom of the glass to bulge up and be concave.
Soak Mold in Water:
Blowing into a dry wood mold will only cause it to catch fire. The mold has to be soaked in water before blowing. After each use it's put back in the water until it's ready to be used again.
Step 3: Blowing
After the mold has been sucking up water for at least a few hours, it's ready to use.
Blowing is done by collecting a few gathers of glass onto a blow pipe, blowing a bubble, necking in with the jacks, and then gently blowing into the mold while rotating.
Burning In the Form:
The first few pieces won’t be perfect, but as the mold burns in, any imperfections in the slats will burn away, it forms a layer of carbon, stops burning, and the rest of the pieces will be relatively round.
Make sure the glassblower knows to TURN when blowing into a mold like this. If they don't turn, the glass will just bulge out between the slats.
When pulled out of the mold, the glass cools down a little bit and is knocked off into the annealing oven where it slowly comes down to room temperature so it doesn't crack. The pieces can usually be removed the next day, unless they're very thick. Giant glass telescope lenses, for example, can take a few years to cool down completely.
The last steps are to fire crack the top off and then polish the rim.
Hot Glass Courses and Resources in the Bay Area:
Step 4: Fire Cracking
There are a lot of Instructables that show different methods for cutting bottles. If you have access to a tile saw, that can also work, but it's very difficult to cut thin glass without it blowing out. This is why I prefer fire cracking.
Fire cracking is when you scratch a line in the glass with a diamond or carbide bit, and then rotate the piece while heating it up around the line with a very small but intense flame. The difference in temperature between the cold glass and the hot flame is enough to cause the piece to crack, more or less following the control line that’s been scored into it. This was my first time fire cracking with an old turntable. In the past I’ve always used a banding wheel and rotated it by hand.
The flame should be a very very small point if possible. Focus the heat on the line as the glass rotates.
At some point the top part of the glass will crack. If the glass isn't cracking, take the heat away for a few seconds to let it go from hot to cool to hot again and start the crack.
When you hear that wonderful CLINK, the glass has cracked. Pressing down on the top will help force it to crack off completely. Lift off and don't forget, the pieces will be hot.
Step 5: Polishing the Rim
Polishing is done on a lapping wheel, with a series of sanding discs, or on a water-fed belt sander with a series of belts. Just like sanding wood, you start coarse and work up to a finer grit, getting a decent polish on cork or felt pads with cerium oxide powder.
The roughest disc or belt is used to get the piece level on top and close to the right shape. Progressing to the next disc or belt will take away any scratches from the coarse grits. Rolling the glass at an angle will bevel the rim, helping smooth out any sharp edges.
The final disc is a felt or cork pad used with cerium oxide powder, which helps achieve a very fine polish.
The inside edge of the glass can be smoothed with diamond hand sanding pads.
Step 6: Finished Glass
When the sharp edges are gone, you can use the glass. Taking the rim up to a full polish makes it nicer to use, but this can be time consuming.
The wood mold should be stored in water, but it can be put on the shelf to dry for use another time.
Clean and wash the glasses by hand, and try to avoid exposing them to big changes in temperature (going from hot to cold, or cold to hot) because this can cause them to crack.