How to make home-made calcium blocks for tortoises

Tortoises need calcium in order to build strong bones, a strong shell, and healthy organs. Ideally, they are fed a widely varied diet of nutritious weeds and dark leafy greens that have high levels of bio-available calcium in them. Adding a sprinkling of a healthy plant-based supplement such as TNT can help significantly. However, sometimes a tortoise's calcium requirements go beyond what is found in the plants we can provide. 

Some tortoise keepers rely on sprinkling calcium powder on tortoise food. However, this has the danger of causing a dosage beyond what a tortoise needs, and may actually cause health problems. For this reason, I like to provide healthy tortoises with calcium sources that they can help themselves to, as needed. Tortoises seem to have a good instinct about when their body needs more calcium.

Some tortoises readily eat cuttlefish bone on their own.

Cuttlefish bone (found in the bird aisle of most pet stores, or available in bulk online) is a good option. However, not all tortoises seem to care for cuttlebone. Quarry chalk is a good source of calcium. It is widely available in the UK and some other countries, but is hard to come by here in the US. Finally, man-made calcium blocks are another good source of calcium. Some such products are available in the pet trade. However, I have found them to be ridiculously expensive, and they often have undesirable ingredients such as sugar or artificial dyes and flavors. For this reason, I did a little research, and decided to try making my own calcium blocks.  

The home-made calcium blocks are tortoise-approved!

After seeing how ridiculously easy it is to make the home-made calcium blocks, I wanted to share the recipe and procedure with you, so that you and your tortoise(s) may benefit from it as well. 

First, you need to decide on the calcium source. Food-grade calcium carbonate (CaCO3) is one good option, sometimes called Limestone flour (available e.g on Amazon). Another, more affordable and more easily available source is so called Agricultural Lime (CaCO3) - I got it at our local feed store for $8 for a 50lb bag! Agricultural lime is produced by pulverizing limestone or chalk, and is commonly used as a soil amendment. Please make sure to read the label CAREFULLY: you want to make sure that it not so-called hydrated lime, that it does not contain Magnesium Carbonate, and that it does not have any other ingredients and impurities in significant amounts. Here is the kind I bought:

One of many good options

The only other ingredient you will need to make lime blocks is water.
For tools, you will need:

• a large mixing bowl 
• a sturdy spoon to mix with 
• and some kind of mold that will withstand 200 degrees F (93 degrees C). I used a silicone muffin pan that I got at Goodwill for $2 
• an oven, pre-heated to 200 degrees F (93 degrees C)

It took a little bit of mixing and adding to find just the right consistency. Here is the ratio of ingredients that I found to work best:

• 1.5 cups of warm water (H2O)
• 7 cups of CaCO3 powder (Ok, sue me; I used a liquid measuring cup to measure the dry powder, which I know is a no-no in cooking... but in this case it worked fine, since I was just aiming for keeping track of the ratio that worked)
  
  NOTE: Today I made the calcium blocks again, and this time it took 4.75 cups of CaCO3 and nearly 1.5 cups of water. You may have to experiment just a tiny bit - the mixture should be mixable with a bit of muscle, but not all crumbly. It definitely should not be wet or soggy.

Calcium Carbonate powder

First, I measured the Calcium Carbonate Powder into the mixing bowl. Then I made an indentation into the powder, and slowly poured the warm water into it. Then I very carefully stirred the mixture, to minimize clouds of powder flying up and all around. The mixture quickly gets very thick, so you really do need a sturdy spoon made of metal or wood. A spatula didn't cut it.

Yum! Calcium paste!

Keep on mixing until there is no more liquid on top, and no more pockets of powder at the bottom. It will get progressively tougher to mix, and will start looking like mortar or thick plaster of Paris.

Next, spoon the Calcium Carbonate + water mixture into the molds you have chosen. The amount I made was exactly the right amount to fill all 12 wells of the muffin pan, plus 1 small cup.

NOTE: Please DO NOT dump excess slurry down your drain. It can clog your drain and/or wreck your in-sink disposal. I took the bowl outside and hosed it down. It is perfectly safe just to dump it onto your garden or your lawn. 
In fact, if you don't have a lot of tortoises, and don't want to have the remnants of a 50lb bag (minus 7 cups) of agricultural lime sitting in your garage... just spread the rest into your lawn, or dig it into your flower beds or raised garden beds!

Soon-to-be Calcium blocks

Next, carefully place the mold into the oven. I used a cookie sheet under the silicone muffin pan since it is a bit wobbly. Set the timer to 1 hour.

Bake the calcium cakes

After 1 hour of baking, open the oven door and carefully pull the oven rack out far enough that you can safely touch the calcium cakes. You will notice that water has risen to the top of them. Use a paper towel or napkin to wick off the excess liquid. This will significantly cut down on the drying time.
Bake for another 2-3 hours, depending on your oven. I checked every hour to remove more liquid, and after a total of 4 hours, they looked completely dry, and had shrunken in the molds.

Carefully remove the mold(s) from the oven, and place somewhere to cool down. This took a long time. I used our handy dandy infrared thermometer to check the temperature of the calcium blocks, and waited for them to almost be cool enough to touch.
Once they are cool enough, carefully remove each calcium block from the mold, and place it on a cookie drying rack. They should come out easily. I let them sit like that overnight, just to make sure all the moisture had dried out.

12 beautiful calcium cupcakes.

The next day, I put one of these beautiful calcium cupcakes into each of the tortoise enclosures, and packed the extra ones into ziploc bags for later use. Within minutes of placing the cakes into the enclosures, the tortoises congregated around them, and took a few nibbles. Pretty soon, they were sporting cute little calcium mustaches... 

Jill approves.

The part that really made my day: Amber, my XXL female Russian tortoise had just laid eggs. She has been stubbornly refusing to eat cuttlefish bone, and does not eat weeds if I have sprinkled them with calcium. However, by evening, her calcium cake looked like this:

Amber loves her calcium cupcake!

I hope you have fun making your own calcium blocks, and that your tortoises enjoy them as much as mine do! Please keep in mind that in order to properly absorb calcium, a tortoise needs UVB to produce vitamin D3. You can read a little more about this HERE.

Ps: When you mist your enclosures, please avoid spraying the calcium cakes too thoroughly (you might take them out briefly), because otherwise they may disintegrate.

Pps: You might experiment with mixing in ground up weeds, or a little bit of carrot juice for a nice orange color... however, my tortoises (even the ones who NEVER eat cuttlefish bone!) heavily approved and helped themselves to these calcium cupcakes within minutes of placing them into each enclosure.

Ppps: I am not the inventor of home-made calcium blocks. There are several conversation threads on the Tortoise Forum (e.g. HERE and HERE) that mention them long before I ever thought of making them. They don't however, mention the exact ratio of water and CaCO3 (Calcium Carbonate) that works best.