I'll start by giving you the recipe.
The Earth Discoveries dig block mix is 6 parts fine washed sand, 1 part Plaster of Paris and approximately 1 teaspoon of yellow iron oxide pigment.
It's actually a really simple mix.
But it definitely didn't start that way, and getting to this point wasn't as easy as I expected.
When I first started developing the Earth Discoveries excavation kits, one of the biggest things I wanted to get right was the actual digging experience. I'd seen and tried excavation blocks that were incredibly hard to dig through, and I didn't want kids spending half an hour attacking a block just trying to make a dent in it.
I wanted something that felt solid, but could still be excavated relatively easily using the small tools included with the kits.
That started a lot of experimenting.
Starting With Plaster
My first thought was the obvious one: Plaster of Paris.
It was easy to cast, readily available and set into a nice solid block.
There was just one fairly major problem.
It was rock hard.
For something that's supposed to be enjoyable to excavate, solid plaster wasn't going to work. It took far too much effort to break apart, particularly when the excavation kits were going to be used by kids.
So I started adding sand.
Testing a 50/50 Sand and Plaster Mix
The next experiment was roughly 50% sand and 50% plaster.
This was definitely heading in the right direction. The sand weakened the plaster and made the block easier to excavate.
But it was still far too hard.
At this point, I realised that finding the right mix was going to take a bit more experimentation than I'd originally expected.
And that's when things got slightly more complicated.
The Mix I Thought Was Going to Be the One
Eventually, I came up with a completely different recipe:
3 parts cornstarch
3 parts perlite powder
1 part sand
1 part plaster
Initially, I thought I'd cracked it.
The consistency was great, and most importantly, it was easy to excavate.
That solved one of the biggest problems I'd been trying to address from the beginning. You could actually work your way through the block with the excavation tools without needing to smash it apart.
But there was another problem.
It looked terrible.
The mixture produced a pale, off-white block that didn't look anything like the material you'd expect to excavate a fossil from.
Finding Yellow Iron Oxide
That's when I discovered yellow iron oxide pigment.
Adding only a small amount completely changed the appearance of the blocks. Instead of an artificial-looking off-white block, they had a much more natural sandy, earthy colour.
At that point, I genuinely thought I'd finished.
The blocks looked good.
They were easy to excavate.
They held together.
Problem solved.
Except it wasn't.
Back to the Drawing Board
After the blocks had been sitting for a while, I noticed something wasn't right.
They had developed a strange smell.
My suspicion was that the cornstarch was responsible. Because cornstarch is an organic material, I wasn't comfortable continuing to use it when there was a simpler alternative.
I also wasn't particularly happy with the amount of fine dust the blocks produced during excavation.
The likely culprit there was the perlite powder.
Even though the digging experience itself was good, I didn't want to sell something I wasn't completely happy with.
So the entire recipe was scrapped.
Back to the drawing board.
Making It Simple
By this point, I'd learned quite a bit from the failed mixes.
I knew plaster worked well as a binder.
I knew sand helped control how hard the finished block became.
And I knew the yellow iron oxide produced the appearance I wanted.
So rather than trying to make the recipe more complicated, I decided to make it as simple as possible.
Sand and plaster.
I bought a 10 kg bag of fine, washed and dried sand from Bunnings, picked up some Plaster of Paris suitable for craft use, and spent a few hours in the garage making sample blocks.
Different ratios.
Mix.
Cast.
Wait.
Break them apart.
Adjust the ratio.
And repeat.
Eventually, I landed on the mixture I'm using today.
The Earth Discoveries Dig Block Recipe
If you want to make your own excavation blocks, here's the recipe I've settled on:
6 parts fine washed and dried sand
1 part Plaster of Paris
Approximately 1 teaspoon of yellow iron oxide pigment per batch
Water as required
The exact size of your "part" doesn't matter. You could use cups, scoops or any other container. The important thing is maintaining the 6:1 ratio of sand to plaster.
For example:
6 cups sand + 1 cup plaster
or
3 cups sand + ½ cup plaster
Start by thoroughly mixing the sand, plaster and yellow iron oxide together while everything is dry. This helps distribute the pigment evenly and gives the finished block a consistent sandy colour.
Gradually add water while mixing until the mixture is wet enough to pack firmly into your mould. You don't want to add more water than necessary, as that just means more moisture has to come back out again later.
Once packed into the mould, allow the plaster enough time to set before carefully removing the blocks.
Drying the Dig Blocks
This turned out to be another important part of the process.
Even when the plaster has set and the block feels solid, the large amount of sand can hold a surprising amount of moisture. Leaving the blocks to air-dry works, but it can take quite a while.
For my blocks, I speed up the drying process by placing them in the oven at around 60–70°C for approximately two hours.
This is a drying step rather than part of the initial plaster set. I let the blocks set first, remove them from the moulds, and then use the low oven temperature to help drive out the remaining moisture.
After drying, I let them cool completely and check that they're properly dry before packaging them.
If you're trying the recipe yourself, drying time will vary depending on the size and thickness of your blocks, how much water you added, your sand, and your oven, so two hours should be treated as a starting point rather than an exact rule.
The end result is a solid, dry excavation block that's hard enough to survive handling but still easy enough to excavate with simple tools.