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Discussion Starter · #1 · (Edited)
In 2008 Aaron Talbot described the process of creating mineralized substrate. The thread it generated contains a number of questions about the clay part of the formula. Being new to APC I missed out on the conversation, but as a potter I have a few tips that will still be helpful which include...
  1. What kind of red clay to use
  2. Some suggested brand names
  3. How to find local pottery suppliers
  4. A word about dolomite
  5. How to break down clay in order to add it to the mix


Keep in mind that all red clay contains iron (Fe2O3). That is why it's red. In its raw form Fe2O3 is a fine powder, also known as red iron oxide. For the purposes of mineralized substrate the issue you want to address is how much iron is in the clay. A good range to shoot for is 3.5% to 7.0%. Many pottery clays are in this range.

When it comes to pottery clay don't worry about whether your clay is low-fire, medium-fire, or high-fire. You are not going to be firing it in a kiln. But just so you know, Earthenware is commonly known as low-fire, Stoneware is generally both mid-range and high-fire, and both of these clays will do fine for mineralized substrate. If you hear the term clay body is potter-speak for the profile of the clay, whether it contains iron or no iron, sand or no sand, and so on. There are many variations and each variation is a clay body.


IMCO - Made in US - Store Locator
Banta Red Clay - 5.9% iron
Imco 800 Fireclay - 6.44% iron
Lincoln 8 Clay - 5.4% iron

Laguna - Made in US - Store Locator
Newman Red Clay - 5.58% iron
C-Red - 12% iron - basically the same clay as Carbondale. This may be too high for mineralized substrate at the levels recommended by Aaron Talbot. Its probably wise to cut back on the quantity if you use it.

Plainsman - Made in Canada - Store Locator
M2 - 6% iron
M390 - 3.9% iron - I used this clay for 12% of my substrate, which should work fine as a trace element.
M332 - 5% iron

REDART - 7.05% iron - an excellent choice because its widely available and easy to get. Pottery suppliers commonly carry this product in dry form, and sell it in small amounts. It can be used in glazes as well, so you may find it in the dry goods section. Its made by Resco, and goes under the name Cedar Heights. Click here for a product shot.

Let me know if you have discovered another iron clay, and I'll add it to the list.

ON-LINE - Clayworld has 3 great dry clays. They sell small quantities of
REDART - 7% iron
Newman Red Clay - 5.58% iron
Carbondale Red - 12% iron


Google "Pottery supplies + your location". That's probably the fastest way to find local suppliers.

When you connect with a supply store keep in mind that sometimes they carry certain brand names at the exclusion of other brand names. An employee may able to quickly help you find red clay, but, knowing the exact percentage of iron will likely be beyond their everyday working knowledge. You could benefit from emailing the store first, asking them which high iron clay they carry, then research them a little on the internet before you make your trip to buy the clay. If you want to use one of the clays listed above, they might have to order it ahead of time. You should expect to pay between $15 and $25 a box for clay at a pottery store in Canada and the US. I am not familiar with dry clay prices, nor with prices in other parts of the world.


Pottery + your location
Pottery club + your location
Pottery association + your location

Your local potter is the person who has the most real-world knowledge that is relevant to you, and they love to share information. With a little effort I think you will be able to find a local potter who will either…

  • Sell you a small amount of clay for a few bucks (¼ pound per square foot of tank bottom)
  • Suggest another potter who uses red clay (not all potters use red clay)
  • Pass your request by email to their pottery club/guild/group
  • Suggest a red clay they use
  • Tell you where they buy their supplies
  • Perhaps sell you some dolomite as well (more on dolomite below)
Many potters and pottery groups have websites with phone numbers and email addresses on those websites. Write a message like this, or maybe just copy and paste it.

Hi - I have an unusual request that I hope you can help me with. I`m looking for some red clay for a project I`m working on, and was wondering if you have a small amount to sell, or if you can suggest a colleague who uses it, or a store that stocks it. I only need a few pounds, dry or moist, at any cone temperature. Any help would be appreciated. Thanks.

The ``cone temperature`` is just the temperature the clay is fired to. Different clays mature at different temperatures, but that will have no bearing on your project since you won`t be firing it.

  • Porcelain contains no iron.
  • Sculpey and Fimo have polymers added in, and are not considered to be true clays. Stay away from these polymer based products. They are not beneficial to plants and fish.
  • Modelling clay is a term that has taken on a lot of meanings. To avoid confusion, stay away from it. There are plenty of other clays to choose from.
  • Local clay you find by stream beds and in your yard can be suitable for mineralized substrate, but keep this in mind; the clay in my area is high in sulphur, so unless I am prepared to clean it, I am much better off buying prepared clays because the manufacturer has already cleaned them. Sometimes clay comes out of the ground in beautiful shape, but I would only know whether its safe by having it tested, or if there is a strong tradition of local use, as in the state of Georgia. If you really want to know, ask a local potter.
  • Red soil that has a lot of iron in it is not something I am familiar with. I read an article by Steve Pushak in which he stated clay helps increase cation exchange. So, under that theory, you still want clay in your substrate.


While you are at the local pottery store you may as well pick up some dolomite. The dolomite potters use is ideal for mineralized substrates, and its cheap. I use a local brand, Imasco Dolomite, which has the following profile.

CaCO3 - 55.0%
MgCO3 - 43.0 %
Acid insolubles - 1.4 %
Ferric oxide Fe2O3 - 0.6 %

As you can see dolomite adds both magnesium and calcium. Different dolomite has different profiles because it is mined form different locations around the world. So if you're curious, when you email the pottery supplier about the clay, ask which brand of dolomite they stock and Google the manufacturer. The manufacturer should have some kind of spec sheet on their product. For example…IMASCO Dolomite


If you buy dry powdered clay you`re laughing. All you need to do is mix it into the soil as Aaron described in Step 5.

If you buy moist clay. There are many ways to go about adding clay. You can tear off pieces and throw them directly in the mix in step five. But it`s probably best to prepare them well ahead of time so you don't slow things down when you get to step five.

I recommend this method, which should about 15 minutes, plus drying time.

  1. Pre-weigh your clay to ¼ pound per square for of tank bottom, or estimate it at 10% of your soil.
  2. Place a cardboard box on your lap.
  3. Grab a lump of clay, about the size of an apple.
  4. Pinch off small pieces no larger than a single peanut and drop them into the box. They can be any shape you wish. Flat discs will work. Don`t worry about how they appear. The smaller the better for wider distribution in the substrate.
  5. When the apple sized chunk is gone, grab another, and create as many of these little pieces as you need. You can let them pile up on top of one another if you like. If they decide to stick together a little don't worry that for now. They will come apart much better later if you just leave them alone.
  6. Let them dry in the box in a sunny spot, or on a heater. You can place them on a shelf at room temperature and wait a few days. Putting a fan on them will speed things up a lot. The moving air will suck the moisture out of the clay balls.
  7. When they are bone dry you should have a bunch of little brittle pieces of clay. If they are still sticking to one another just rub them together and they will detach quite easily.
  8. To get them smaller bash them with a hammer inside a box so the pieces don`t go flying everywhere, or put them on a board and roll over them with a rolling pin and they should crumble a little more.

I hope this helps clear up a few questions about the kind of clay to use, where to get it, and how to prepare it.

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Resurrecting an old thread!
I read somewhere on this forum that Diana Walstad advises not to use red clay as the iron causes algae blooms. Has anyone found this to be true?

Sent from my Nexus 6 using Tapatalk
I don't know for sure about algae blooms but I do know that exposed pieces of laterite in my tanks like to grow bba attached to them. I have read many many places of people taking red clay and making little balls to cover the bare bottom before putting the substrate of choice on top. Hope this helps, maybe others can help more

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Both redart and laterite work fine in soils. Personally, I usually used them as a base-layer underneath soil rather than mixing. To be honest though, I've never noticed enough difference with or without it and lately I've been thinking of not bothering to use it anymore, just because it's such a pain to work with.

The recommendation against it in Walstad's book was based on a handful of experiences (actually, only two of her failed aquariums, if I'm not mistaken. It's late and I don't really feel like double-checking right now).

It's important to remember that as awesome as that book is as a basic introduction, most of that book is written from personal experience, and only uses citations of actual scientific literature to try to form an educated best-explanation.

Most of her conclusions are sensible, but I'm sure in hindsight she'd reconsider some parts...

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Why not add a small amount of Fe203 with the Dolomite and Muriate of Potash as a bottom dressing? Or possibly white iron ferrite for use with silica sand? I have bottom dressed silica sand with good results and would like to add iron for red plants. I sell various substrates and clay is always somewhat of a problem.
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