Repairing a Broken Mould

Today’s chatter describes an attempt to repair a broken ceramic mould so that it can be brought back into useful service. The repair process takes time and patience but it might help you salvage a expensive broken clay mould.

If you’re from the USA then today’s extra learning is that when I write the English word “mould” you should pretend you saw “mold” and not confuse it with the Welsh town called Mold.

Some Background

Many years ago, when I had more teeth and my hair was pigmented, I was still naïve in all matters relating to fused glass. I bought a little teardrop mould that would fit into a recently purchased microwave kiln thinking that it would be fun to make some fused glass teardrops.

What I didn’t appreciate in those days was that a microwave kiln is a harsh beast with rapid heating and cooling that was going to be a problem for the mould. All I can say in my defence (or defense in you’re in the USA) is that there was no information sheet with the mould giving me a recommended kiln schedule which might have served as an implicit warning that trouble was ahead.

As I now know disaster was entirely predictable. The mould cracked into three pieces on my very first attempt to use it. So, for several years, the broken mould languished unloved but not forgotten in its box until I could figure out what to do with it.

Hindsight is a good teacher and lucky for me it was not an expensive mould.

The Underlying Idea

I know from previous experiments that a glass kiln may not be able to reach the high temperatures required to fire clay to completion, to the satisfaction of a potter, but can be processed in a glass kiln sufficiently to make a usable mould. The resulting moulds were not as strong as a “proper” ceramic mould but have shown themselves to be perfectly adequate for use with glass.

I therefore reasoned that it might be possible to use some clay slip (of thick double cream consistency) to “glue” the broken parts of the mould back to together then strengthen the joints by firing the repaired mould in my glass kiln. I was right.

The Outcome

DSCF3312 Mould RepairHere’s a picture of my repaired mould. It is a small Slumpy mould and has been repaired by the method I shall now describe for your. You can still just about see where there used to be cracks and you can see I still need to give it a final clean-up.

You’ll have to take my word that it’s now good enough to be brought back into use!

I hope you have equal success but if you have any extra good ideas to contribute then please let me know and I’ll update this blog with your new information.

The Repair Process in Outline

If you’re in a hurry or can’t cope with my copious chatter, here’s a summarised description of the repair process.

  • Clean mould, removing kiln wash and other contaminants
  • Wet mould then use thick clay slip as a glue
  • Clean away excess clay carefully
  • Allow mould to dry, using elastic bands (etc) to hold pieces in place
  • First firing to strengthen the repair joints
  • Second round of repairs with clay slip to address remaining defects
  • Second firing to make mould as-good-as-new

Read on if you’re still interested!

The Repair Process in Detail

The first step in the repair process is to thoroughly clean the mould, attempting to remove all traces of kiln wash, dust and grime. For this I used a single-sided razor, a kitchen scouring pad and copious amounts of water. In effect, this task is much like cleaning a kiln shelf before re-applying kiln wash. If you’ve unfortunately succeeded in dropping your mould before it was ever used then this step will not be needed.

If you’re previously prepared the mould with a boron nitride spray then you will not be able to completely remove the boron nitride. Do not despair. Try your best and stay hopeful as the coating should only be at the outside surface and not at the edges that will be “glued” with clay.

You will need some thick clay slip. I used exactly the same cheap clay for the repairs as I used for my previous mould-making experiments even though the clay was supposedly not suitable for use in a kiln! I mixed just enough water and some clay scraps to form a smooth gloop with a consistency of thick cream.

When you have a nice clean mould, pop it into a bowl of water for a few minutes. It is going to be easier (and the results will probably be a little stronger) if you “glue” the pieces of mould together when they are wet because dry pieces of mould will rapidly such the water out of the clay slip making it too stiff to accurately and neatly “glue” the pieces of together.

Next comes the gluing step. As you would with ordinary glue, apply a thin layer of your clay slip around all the edges that need joining. Gently push together all the broken pieces, trying your best to accurately place all the pieces together. Now is a convenient time to wipe away the worst of the excess clay slip that squeezes out. This step is made easier by having previously wetted the mould pieces!

Do not worry too much if a few little shards of mould have been lost, or the some of the pieces refuse to sit exactly where they ought to. This can all be resolved later!

With all your pieces of mould “glued” together it’s time to ensure that nothing moves until the mould has dried off. I used elastic bands but any other method that keeps everything stable can be used. Tight joints are what we are aiming for.

At this point a little gentle washing with a damp sponge (or fingers) can be used to remove some more of the excess clay. But be careful you don’t cause the clay mould pieces to fall apart or move. Do not worry if you can’t clear away all the excess. This can be done later.

Now leave the mould until it is completely dry. I think natural drying is the best choice because it allows clay minerals to “migrate” into all the tiny gaps which we are hoping will result in a stronger repair. Another reason is that using damp sponges and cloths in the next step are a little less prone to cause the repair to fall apart!

When the mould is dry, remove the elastic bands (or whatever you used) and inspect the mould. If any little holes need filling then now’s a convenient time to fill the holes. Leave the mould to dry-out again if you just did more repairs.

If there are any areas where there is an excess of clay then try to remove the mess with a damp sponge, dampened paper, or whatever else you think will work. You may find that dental picks, lollipop sticks and single-sided razors are also useful. Fingers are an amazingly useful tool as well! I remind you to be careful to not cause the ceramic mould pieces to move or fall apart. Unfired clay is not a strong glue and when you add water the clay “glue” suddenly gets even weaker. A strong hint for this step is not to use too much water!

Whenever you “re-work” the mould repair and cause it to become dampened, add those elastic bands and again leave the mould to dry out completely.

Do whatever is needed, again and again, until you are happy that your repairs are about as good as you can get. But do not obsess about joints that are not quite right or that there may be a little hole that needs filling because this can also be addressed after your first firing.

You’ve now got the point where you’ve done your best with the repair. Well done. It’s a messy and fiddly job and you probably had to do it several times to get it right. Patience is a virtue! Now is the time to convert the weak clay glued joints into a stronger mineralised clay joints.

For my first firing I happened to be slumping some bottles and there was an unoccupied space on the kiln shelf to fill, so that’s what I did first. Slumping at around 700 degrees Celsius is hardly a full fuse but I found that even this low temperature was enough to fire the clay enough to begin to mineralised and form an acceptably strong repair. Another advantage of this firing schedule was that it was conveniently slow and gentle.

After this first firing I found that some of the original mould pieces were not aligned exactly. I could feel the joints with my finger nail and decided that they would result in unsightly lines on anything made in this repaired mould. I also spotted a tiny little depression where three piece or mould met. So, the obvious answer was to attempt a “improvements” repair.

The improvement repair was nothing more than to apply a little more clay slip to areas that needed them. Again I wet the mould before applying more clay slip but this time there was no need to worry about the mould falling apart because the joins were already quite strong. Again, you may need to do this several times until you’re happy with your final repair attempts.

With the final repairs done, leave the mould to dry out (yet again) and then put it back into the kiln to produce what we hope will be a good-as-new repair.

Job done. An expensive but broken mould can be brought back into service!

Some Extra Notes

You may be concerned about the strength of the joints and that a ceramic mould is being repaired using clay at a temperature that is much lower than a potter would have been used to produce the original mould. Let me explain my understanding and the consequences…

Although I am not a potter, I am aware that clay undergoes various predictable chemical changes at different characteristic temperatures and that each of these chemical changes happen only slowly. Each of these chemical changes we can think of as progressing the clay from a weak substance stepwise towards a very strong ceramic substance. This also explains why potters use long and slow schedules compared to our “in and out quickly” schedules. Forming a strong crystalline structure is good for pottery but is bad for glass work! Yes, I’m thinking of devitrification!

So, by firing our repairs in a glass kiln at a relatively low temperature and for a relatively short time we only cause the clay repair to incompletely take the first few steps of the pottery firing process. From this I think there are three practical consequences:

  1. We have not fired the repair to a high enough temperature. This means the resulting repair is not as strong as the original ceramic mould because the clay has not been converted to the same “final” chemical substance that the original ceramic mould is made of. This means we have to handle the repaired mould with just a little extra care.
  2. We have not fired the repair for long enough. This means the resulting repair is not as strong as the original ceramic mould because the clay was probably not in the kiln long enough to allow achievable chemical changes to be happen completely. This leads to an interesting speculation that the repaired mould may get a little stronger after being used a few times. But again, it means we have to handle the repaired mould with just a little extra care.
  3. By under-firing the clay minerals of the slip have not become as hard and durable as the original mould which, conveniently for us, means that our repair is still soft enough be sanded or scraped to address remaining defects.

When All Else Fails

If you’ve done your best but the repair fails or is not to your satisfaction then I offer a final thought…

If you can repair the mould sufficiently to achieve a close approximation to the original mould then you have something that could be used to make another mould. As they say, there’s more than one way to skin a cat.

Go for it. At the very worst you start with a broken mould and end up with a broken mould!


About chatterglass

Maker of stained glass frippery.
This entry was posted in Experiment, Microwave kiln, Money-saving ideas, Mould, Recycling, Repair and tagged , , , , , , . Bookmark the permalink.

2 Responses to Repairing a Broken Mould

  1. Interesting post. Having done some clay work, I would think that it might be helpful to put a little worm of clay along the back of the crack. However, I really have no idea. I actually wrote this I meant to tell you of my experiences with a cone 10 clay that I tried to make some Kiln shelves from. I have a small Kiln that will actually fire to 2000 degrees. I made some Kiln shelves approximately three-quarters of an inch thick, dried them extremely thoroughly and fired them to bisque cone 06. I thought this would be enough to make them a functional shelf since I have heard you can use bisque ware as molds. When I used them to fuse glass to 1450 they developed hairline cracks which subsequently caused them to break into two pieces. This isn’t a bad thing, I now have more smaller shelves which are actually more useful. But I wondered what you thought of this and if there was something I should do differently? I didn’t want to fire them fully because when I fired one fully it warped.

    • chatterglass says:

      Hi and sorry for the delay replying. I’ve had a miserable time recovering a poorly computer!

      I like the idea of using a little worm of clay for strengthening a crack so will try it.

      I don’t know much about clay but will speculate – If your kiln heats unevenly or heats-up too fast or cools-down too fast then induced stresses may be be the cause of the hairline fractures which ultimately led to breakage. That the smaller pieces are not breaking-up into yet smaller pieces suggests that they are now too small to build up enough stresses to fracture again.

      This reminds me of my little microwave kiln – when cold I see several hairline fractures. It too is subjected to fast heating and cooling. This seems to corrrelate nicely.

      So, maybe try again but with much gentler heating and cooling?

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