Concrete Geode

This project would make a good introduction to working free form & playing with concrete. It is simple enough to be a beginners project and hopefully has enough scope for people to make it more or less complicated so please feel free to experiment or adapt the process depending on what you need.

There are a few optional changes in case you find any of the materials difficult to source or afford. I'll add them into notes as we go.

Geodes are amazing natural occurrences. Mineral rich hydro-thermal fluids become trapped in volcanic or sub-volcanic cavities, the outer rock hardens and the minerals "grow" into the crystal structures commonly associated with geodes. Geology.com has a really good article if you want to learn more about the formation process of geodes here

Although this could be an educational project, bare in mind that concrete and resin have their hazards when working with from corrosive to flammable. Breaking glass with hammers and thermal stress is a hazard all of its own also so I will begin the equipment list with safety first...

You will need

PPE

Safety Goggles
Disposable gloves (several pairs)
Respirator/ face mask
Surface protection/ dust sheet
Clothing protector

Materials

Quick setting concrete
Decorative glass nuggets
Acrylic paint to match your crystal bed
Gold or metallic paint (or marker pen)
Clear casting resin + hardener (or any clear setting glue)
Resin colorant (or transparent paint that can be mixed with coating glue)
Hot glue (or glue that will bond glass)

Equipment

Glue gun (if opting for hot glue)
Mixing jug/ bowl
Measuring cups, mixing spoons, spatulas etc.
Paintbrushes including either and old one or dedicated dry-brushing one
Hammer
Baking tray
Wire brush
Ice cubes (optional)
Kitchen foil, scrap paper, old plastic bag
Scissors, tape, usual crafting bits and bobs

Teacher Notes

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Step 1: Forming the Form

Did you know that it can take millions of years for a geode to form? If you don't have that sort of time then here's where to start...

Take your scrap paper and form it into a loose ball shape. This will form the hollow area inside the concrete where the "crystals" will be bedded.

Wrap this in kitchen/ aluminium foil and then that all into a plastic bag. The foil helps the paper to keep its shape so you can modify the shape as much as you need to at this stage. But make sure that once you have finished wrapping in foil the ball is less loose to prevent the concrete weight collapsing it.

Unfortunately, the concrete sticks to the foil (trust me, I've a lot of foil lined lumps of concrete in the house!!!) which is why you need the plastic bag.

The size of the shape isn't critical but be mindful of how much weight it will have to support. The only other shape issue to be conscious of is how steep the sides are. You don't want the concrete it sliding down before it has time to cure! If you want to make a much bigger version, thought should be given to including a reinforce to the concrete mix (fabric, sawdust, paper shreds are all viable options)

I covered a piece of scrap card in plastic also, so i had a more movable surface to add the concrete to.

Tack your ball form to the surface. It doesn't matter if there is a gap underneath.

Now we move onto the messy bit...

Step 2: Concrete

When mixing your concrete if found that I had better results if I used about 5-10% less water than recommended by the instructions on the packaging. Be aware however that it also shortens the amount of time you have to work the material. Also, the warmer you room is the faster the concrete will cure. Mix wearing a mask because concrete dust is nasty!

Wearing your gloves... start by taking walnut sized blobs and adding repeatedly to cover the whole paper form. Once that's done start adding to form the shape you want the outside of your geode to be.

Make it rough and irregular! DO NOT TAP! this causes all the concrete to liquefy causing smoothness and sliding off the form.

Once you are happy with the shape, set this aside because its time to break stuff...

Step 3: Making Crystals

When broken through heat stress, decorative glass nuggets seem not to have too many super sharp edges (although I can't guarantee this so conduct this step at your own risk!) Another option would be to break a sheet of tempered un-laminated glass but this would only work for projects that involves crystal beds with a green/ blue color cast. Again, I cant be 100% that it would be safe. Faux crystals or the plastic version found in pet stores are the only way to be 100% safe here but look a bit too regular in shape for what I was aiming for.
Wear goggles, gloves and be careful around extreme heat/ cold

The sharp risk is also one of the reasons for the resin/ glue pour discussed later.

Now that I've gotten my health and safety brief done on to the breaking...

Decorative glass nuggets are sold in many cheap home stores for putting in the bottom of candle holders/ fish tanks. They come in a range of colors so experiment based on your paint scheme and geode ideas.

Pour all of the glass into a thin layer on a baking sheet. I put them in as the oven was warming up to reduce the risk of them cracking and jumping about the oven. Gas mark 5 for 20 minutes seems to work well.

While this is heating and baking, fill a large bowl with water as cold as you can get it. I added ice because it was pretty hot but it does not take a huge swing in temperature to stress glass to the point of fracture.

Once the 20 mins is up you need to move as fast as possible to pour the hot glass into the cold water. The close up pic shows the internal fractures formed by this. Without doing this step, I found the nuggets too difficult to break into anything other than powder.

Once cooled, dry them off then using an old piece of fabric and tap each one with a hammer on a hard surface. Wrapping them in the old fabric contains most of the bits. Each one seems to break into 4-5 bits with only a little glass dust. Put the broken nuggets aside and hoover the heck out of your work space!!!

*I am proud I've resisted as many hammer puns as came to my head as I was writing this

Step 4: Final Shaping

Peel the concrete blob off the backing. I left mine for about an hour so It was still a little brittle but no longer liquid. After opening the tape on the back of the form, and pulling the inside out in layers I was left with a hole that was a little too small compared to the idea I'd had. Pushing the thinner areas caused them to snap off and then use a wire brush to finish the shaping and adding of texture on areas that were a bit too smooth

Step 5: Painting

Leaving the geode to dry for another half an hour to an hour after the previous step helps the paint stay nice and bright rather than bleeding into the concrete. While your waiting for full drying to occur, take some time to plan your color and what type of geode you are aiming for.

I'm lucky enough to have a lump of amethyst that I chose to model on but I've also put together a Pintrest board for you to see some other examples of geodes...

My geode inspo board

I started with white priming then added and blended blues and purples. Don't forget that the indent where the 'crystals' will go disguises any paint mistakes when painting in a small confined space. Don't try to make that area perfect! I don't paint it all the same shade however as with clear glass the colors mottling underneath looks better than a flat color. Adding gold using paint or a marker adds interest. I also filled bubbles left from adding the concrete with gold and trimmed the edge on the reverse.

The reverse was painted with a dirty verdigris and then dry brushed to highlight the roughness left from wire brushing or slopping the concrete on

Step 6: Adding the Crystals

Glass is quite difficult to glue in place but broken edges help give the glue something to purchase. I used hot glue but epoxy or all purpose strong glues work just as well. Wear gloves if handling the glass rather than pouring it into the hole

For the hot glue, I added small batches and spread it about with a wooden popsicle stick. I then poured the glass chunks in, let the glue go cold, poured the excess back out and repeated until the entire inner area was covered. If you are using hot glue make sure to have the gun on the hottest setting as the concrete and glass suck heat fast.

If using a slower setting glue, paint a generous layer on the whole cavity and then pour the glass nuggets in full to the rim. Allowing the glue to set while the entire hole is filled with glass stops everything from pooling in the bottom.

Step 7: Final Fixing

I used clear resin to finish the surface and fix the glass super securely. It also coated any small sharp parts of the glass. Any clear glue such as UHU general purpose or clear PVA (modge podge) has also worked.

Adding colorant to the resin (or glue) gives a bit more interest if wanting the crystals to have a touch of color.

I mixed 30ml of resin according to the instructions using a 2% hardener mix and slowly adding the color until I was happy with it. Resin is hazardous to your health, use protection and follow whatever safety data your brand suggests

Pour whichever mix you choose around the rim of the cavity and swish it about until all of the glass has a layer. You can also use a paintbrush to move the resin/ glue about.

Painting the colored surface adds a solid varnish and covering the whole geode with resin (or marine varnish) would protect this is it was to be used as a garden ornament

Step 8: And Done

Thanks for reading! Please let me know if you try this or what successes you have! And if you like it please hop on over to the concrete competition and give me a vote :)

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    3 Discussions

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    MadeByBarb

    Tip 23 days ago

    Cool! Tip: hot glue does not attach the glass effectively, the best permanent glue is 2 part epoxy. Keeping the colours natural will give these quite a realism. After inventing the idea; I have been making these for a few years now...

    DIY-Giant-Concrete-Geode--madebybarb-41.jpg
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    DameMolly

    26 days ago

    This is brilliant, and the process is explained so thoroughly. I would love one like this for myself!

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    WeTeachThemSTEM

    26 days ago

    Your geode turned out great! I love all the details you achieved with the paint and resin. :)