Tutorial: Make a Mold from Fiberboard

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Makers of fused glass know all too well how limiting it can be to not have the right shape of mold when you need it. I’ve been there and that's exactly why I tried my hand at making my own. I had tried making molds from clay yet in the past, but I was unsuccessful at it. (Although it is something I’ll attempt again at a later date.) But did you know that you can also make your own molds from fiberboard?

The idea of creating a mold stemmed from a custom order. I had a 9 inch waved plate mold and needed one the same or similar, for 6 inch plates. After days of searching online and contacting a whole bunch of suppliers and not finding what I needed, I decided that making my own mold might be the only way to have a mold for this custom project.

Getting to the point of making a proper mold was a bit of a challenge. There were many failed attempts at creating a mold to suit my needs. How I Made My Own Slump Molds is the backstory of my failed attempts that brought around me to trying this particular method. And since you may want to make your own mold one day, I’d like to share with you the specifics of how to do that, step by step.

This post contains affiliate links, meaning, I recommend products and services I’ve used or know well and may receive a commission if you purchase them using my link (at no additional cost to you.)

What is fiberboard?

Let’s start with an explanation of what fiberboard is.

It’s a ceramic infused material that needs to be handled carefully for it is easily damaged and can create airborne dust particles that are dangerous to breathe in.

​For your own safety, ALWAYS wear a Niosh rated mask when working with this material.

What you will need:

  • Pencil
  • Ruler
  • Popsicle Stick
  • 1" Fiberboard
  • Small utility knife
  • Extra blades for your knife
  • Chisels - I used small chisels made for clay
  • Cardstock or Cardboard
  • Drill bit
  • Rigidizer
  • Disposable paint brush
  • Sandpaper - fine
  • Niosh approved mask
  • Shelf posts & paper
  • Kiln to fire the mold
  • Boron Nitride Spray or Kiln Wash

Carving the Fiberboard

1. Mark the key areas in pencil.

Starting with a 1 inch thick piece of fiberboard, use a pencil and ruler to mark off key areas of relief. You can use a drafting curve as I did in the photo to outline the center shape.

The 1" fiberboard is anything but inexpensive. Just remember that it has so many potential uses. And, in this case, it will provide you with a custom mold with a nice amount of depth.

​2. Cut the outline of the deepest part of the mold and start removing some of the excess material.

The curved shape in the center of my example was where the deepest part of the mold would be. So I cut along the outline of that shape using a small utility knife. Just be careful! You want this area to be deep, but not so deep as to go through the bottom of the fiberboard.

3. Measure the depth along the edges of this area.

Once this area is more or less cleaned out, it's important to ensure that the inside edges are all even in height. A popsicle stick makes the perfect measuring tool for this!

Why not just use a ruler? All of my rulers are metal and very thin. I found that trying to measure with them was inaccurate as the metal would cut into the fiberboard resulting in a deeper reading than what it really was. Since the popsicle stick was thicker, it was more reliable for measuring the edges, as it didn’t sink into the fiberboard at all.

Place the popsicle stick at various points along the edges will make it clear where further material needs to be removed. Using this method of measuring and various chisels to cut and scrape away the excess material, you'll be able to create an even depth along the edges of this area.

4. make sure the whole bottom is the same depth as the edges.

We concern ourselves with this as it wouldn't be good if the finished plate didn’t sit flat on the table and instead rocked whenever it was touched.

In order to determine the depth of the center, lay something flat across the opening so that you can  use the popsicle stick in the same way you were along the edges. In this photo, I used a piece of glass that happened to be on the worksbench

Remove excess unwanted material where needed and continue in this fashion until it is all the same depth.

5. Start tapering down the sides of the plate by carving out some of the fiberboard.

With the bottom complete, start carving the slope from the outside of the mold to the center. I found that using a utility knife where the blade could extend to 2 or 2.5 inches worked very well. It allowed for a wider slice to be removed with each cut so that the fiberboard had a long taper toward the middle.

Tip: Be sure to change your blade as often as needed. Mine seemed to dull rather quickly. I went through 3 blades on this one mold.

6. Make a cardboard template to help you carve all sides the same.

Once I thought that each side was fairly similar and pretty close to what I wanted, I created a template using stiff cardstock to determine the slope I would have for each side. I made another to span each corner. A cereal box would be perfect for this.

Doing this allowed me to place the cardstock along each edge from the center and showed very clearly where the shape needed modifying. Wherever there was light showing between the mold and the cardstock, it meant that I had to remove a little bit more where it was touching the fiberboard. I worked my way around the mold making modifications as dictated by my templates.

7. Make a small hole.

Once the shaping is done, you'll need to drill a small hole through the bottom of the mold to create an escape for air when slumping glass.​ This will prevent air from trapping under the glass and creating large puffed up areas from bubbles.

8. Apply the Rigidizer and Dry the Mold

Pour the rigidizer into a plastic tray and set the mold right into the liquid. Allow it to soak up as much as it can until the whole piece is evenly wet.

It is extremely important to read and follow the instructions that come with the rigidizer for how exactly to dry and fire your mold. 

Some have flashpoints less than 100 degrees hotter than the drying temperature. When making mine, I ended up using my crafts-only toaster oven as this mold was small and I wanted to control the temperature exactly in fear of setting it aflame. It took a number of hours to dry out in the toaster oven (a​bout 5-6​ total.) I could have just used one of my programmable kilns, however they were both busy firing the blanks for the plates.

9. Smooth the surface by sanding

Once dried, sand the mold to smooth out any roughness or lumpy texture that you don't want there. This will be your last chance to modify the shape of your mold. Be sure you're happy with it before proceeding.

10. Apply more rigidizer and Let it dry one last time

You just need a fresh coat on the surface where the glass will be placed. When working on mine, I used a disposable paint brush to apply a fresh coat of rigidizer just on the top surface. 

Let it dry.

Don't worry, it won't take near as long to dry this time as it's just a light coating on the top.

The one I used was the Fiber Mold Hardener by Hotline.

11. Fire the mold

Now you can fire your mold to cure the rigidizer according to the instructions that come with your rigidizer. You'll fire the mold without any glass on it. Prop it up on shelf posts and lay a small piece of shelf paper between the posts and the mold if they haven't been coated in kiln wash. This will prevent the fiberboard from fusing to the posts.

12. Prep the mold and complete a test fire

You can prep your finished mold using your preferred method of Boron Nitride spray or kiln wash and run a test fire with glass.

I ran a test fire on mine using 6mm clear glass and was happy with the result. A few slight modifications to the firing schedule would eliminate the texture you see in the photo below.

I’d love to hear how this technique works for you and what amazing things you make from your own handmade molds. Let me know in the comments below.

Curious to read about my failed attempts at making these molds and what I tried first? It's all here: How I Made My Own Slump Molds

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