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I tried to cast epoxy resin on a pine cone, and used 1kg total (resin + hardener). I used around 3 drops of dye but the object is too dark now (and not see-through). Is there a way to somehow make it more transparent after it has hardened?

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    Hi Tasos, it might depend on the dye, but I doubt it can be chemically changed to that degree at this point. Is it completely opaque, or could you maybe use it as a lamp, so that the shape suddenly becomes visible after turning it on?
    – Joachim
    Apr 1, 2022 at 13:11
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    What did you do after the cast? What was it cast in? While I don't think this is the only problem I wonder what you did to polish it afterwards? Perhaps some sanding and polishing would make it clearer.
    – Matt
    Apr 1, 2022 at 14:36
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    Some dyes fade a little with exposure to UV light. You could try leaving it outside in direct sunlight for a long time (probably weeks to months). However, many resins yellow from UV, so that might just trade one problem for another. I like Joachim's idea of backlighting it. Resin is expensive, but a lot of projects involve some trial and error, and a learning curve to achieve a desired result. There can be a lot of wastage along the way. It can often help to try small batches to gauge things like how much dye to add, but sometimes you won't see the net effect without an actual full trial.
    – fixer1234
    Apr 1, 2022 at 16:13
  • Thanks for the replies! I haven't sanded or polished it yet. The cast was a juice bottle, here's the result so far (terrible I guess, see first post) Should I sand/polish it and leave it in the sun or do it the other way around to see if it clears up? even if the epoxy gets yellow it could look ok I guess
    – Tasos
    Apr 2, 2022 at 4:53
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    If the surface has a frosted effect, a quick fix is to pour a final coat of epoxy on top. You'll need to sand the existing surface with coarse sandpaper to make it even more rough, or the epoxy will pull away from some areas (which is very visible in the end). You'll need to put the pine cone on a stand where excess epoxy can drip off. And in my experience this works best with an epoxy as viscous as honey (so it doesn't flow off too quickly) or with UV resin you can cure quickly. Have some tool at hand with which to move the epoxy around to cover every spot.
    – Elmy
    Apr 2, 2022 at 7:56

2 Answers 2

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Diagnostics

From the pictures, it looks like the problem is transparency rather than darkness from too much dye. In fact, the tinting is barely visible, so ignore my comment about trying to fade the dye color.

From the sides, it looks a bit translucent rather than transparent. That could be from the dye causing cloudiness because it isn't compatible with the resin, or the resin surface being a little matte. There are tiny spots scattered around that look clearer, which suggests it's the surface.

The top view looks much more transparent (also suggesting it's a surface issue), although it's very distorted. The top surface is very uneven. That suggests that the resin was disturbed after it started to gel and harden. That very irregular surface would explain the distortion.

A cloud of microscopic air bubbles is another thing that can make the resin look translucent. There are a lot of air bubbles at the top, but they are much larger than that. If those weren't introduced by mixing, they may have been trapped in the pine cone, and didn't all escape before the resin started to harden.

If you repeat this, use much slower setting, less viscous resin. Trickle it slowly outside the pine cone so the resin level rises slowly, allowing the air to escape as the resin flows into the nooks and crannies. This kind of project would really benefit from a vacuum chamber.

So what to do now?

I would start by washing it with alcohol, then soapy water in case what you're seeing is just a film of whatever you may have used as mold release. As a further confirmation that the translucency is due to the surface, look at whether it's more transparent when the outside is wet. If the problem is that the resin is cloudy, there's nothing you can do now. But from the pictures, it doesn't look that way.

If you confirm that it's a surface problem, you can either polish it, as Matt suggested, or coat it, as Elmy suggested. But if the problem is the surface, it is extremely superficial. Before tackling Elmy's suggestion, I would just try to buff it, maybe start with one side to determine if that approach will be sufficient. I wouldn't even use sandpaper. If just buffing with a cloth doesn't do it, try buffing it with a very mild abrasive like toothpaste to polish it, then rinse it and buff again with a dry cloth.

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    Yes, I understand now that the plastic orange juice box I used is the reason it's cloudy. I started sanding/buffing and it looks a lot better, will post the result when I finish the project thank you!
    – Tasos
    Apr 4, 2022 at 4:23
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This cast was probably too high for this specific resin and due to the exothermic reaction the resin heated up so much that it boiled. That's why there are gigantic air bubbles in it and the surface looks so distorted. You can see a demonstration of epoxy boiling in this video.

Resin is a complicated material to work with. For perfect results you want to mix very exact amounts of resin and hardener and you want it to set in a controlled way. Once the 2 components are mixed together, they start a chemical reaction that produces heat. At the same time, the reaction is accelerated by heat, which in turn produces even more heat. It gets out of control very fast.

Once the temperature reaches a critical point, the resin starts bubbling and steaming like boiling water and it sets extremely fast (I've had a resin with 24 hours curing time bubble up on me and set in 1 hour once). This steam is very unhealthy and you should put the cast outside and leave it alone there immediately. It's lost anyways... Maybe put an upside-down bucket or anything over it so no animal or other person touches it or breathes in too much steam.

The only way around it is to cast several thinner layers one after the other cured. Resin is a bad heat conductor, so you have to make sure that each layer is thin enough that it cannot even create too much heat. Whether the layers are horizontal or vertical doesn't matter. If you are careful with the amount of dye you cannot even see the separation between the layers. The safety data sheet of your resin should contain the maximum safe layer thickness.

To improve the clarity of your cast you have 2 options: either polish the resin to a mirror finish or coat it with a final coat of resin or clear varnish. Polishing is a lengthy procedure that requires much patience and experience.

A much simpler and quicker fix is to pour a final layer of varnish or epoxy over it. You'll need to sand the existing surface with coarse sandpaper to make it even more rough, or the epoxy will pull away from some areas (which is very visible in the end). You'll need to put the pine cone on a stand where excess epoxy can drip off. And in my experience this works best with an epoxy as viscous as honey (so it doesn't flow off too quickly) or with UV resin you can cure quickly. Have some tool at hand with which to move the epoxy around to cover every spot.

The final step: declare the side with the bubbles as the backside ;)

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  • Thank you, what you described is exactly what happened. Do you mean that the resin/hardener amount was too much in comparison to the size of the cast and that is the reason it set so fast (within an hour as you said)? I used 1kg total as that was the amount needed to fill up the cast. Sanding it and polishing works fine but not sure how many times of the same paper size I need to use before going to the next? I also used a wood saw and cut the sides to re-shape it, might do it on the bubbled part as well
    – Tasos
    Apr 4, 2022 at 9:32
  • Rather than the resin itself boiling (it would be more likely to burn before it could get that hot), I suspect there was some trapped water that boiled off, or just evaporated below the boiling point
    – Chris H
    Apr 4, 2022 at 10:07
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    @ChrisH No, the resin itself heats up enough to boil and produce bubbles. You can see an example of the reaction in this youtube viceo. The mixture gets hot enough to deform the plastic cup 32 minutes after mixing.
    – Elmy
    Apr 4, 2022 at 11:02
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    @Tasos The resin was too thick and heated up too much. It doesn't matter how tall or how wide your mold is, you cannot pour the epoxy too high in one go. You'll have to let the first layer cure completely before you can pour a second / third etc. layer on top until the entire mold is full.
    – Elmy
    Apr 4, 2022 at 11:04
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    @ChrisH Yes, referring to the "hot bubbly stuff" as reagents is probably a lot more precise. And I'm not even sure that the bubbling is actually boiling (or simmering) from a purely scientific point of view. It just gets really hot and suddenly it produces bubbles on its own and it looks enough like a boiling liquid that people refer to the phenomenon as "boiling".
    – Elmy
    Apr 4, 2022 at 13:58

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