High-Temperature Mortar Ingredient
Fire Clay for High-Temperature Mortar
Fire clay is the heat-resistant ingredient in the classic 3:1:1:1 firebrick mortar. Here's what it is, why your oven needs it, which fire clay to buy and where to find it near you.
Quick Answers
- Fire clay is a powdered clay that withstands high heat. It's the same type of clay used to make firebrick.
- In homemade high-temperature mortar it's 1 part of 6: 3 sand, 1 lime, 1 fire clay, 1 Portland cement, measured by volume.
- Buy mid-fire or high-fire clay as a dry powder. 20, 35 and 40 mesh all work.
- Look for it at a landscape & masonry supply or a ceramic & pottery supply.
What Is Fire Clay?
Fire clay is clay that holds up to high temperatures. It's mainly used to make firebrick, and it's sold as a dry powder, usually in 50-lb bags. The texture is like flour or baking soda, and the color ranges from reddish brown to white or gray depending on where it was mined.
You'll see it sold under a few names: fire clay, fireclay, mortar clay and bricklayer's (brick mason's) clay. All of them work in the high-temperature mortar recipe. Fire clay is rich in alumina and silica, the same minerals that let firebrick take the heat of a wood fire.
Why Fire Clay Is in High-Temperature Mortar
Regular mortar is just sand, Portland cement and lime. It's great for a garden wall, but Portland cement is not made for the inside of an oven. Adding fire clay gives the mortar a heat-resistant ingredient with a makeup close to the firebrick itself, so the brick and the mortar heat up, expand and cool down together, fire after fire.
Each ingredient has a job. Portland cement gives the first set, lime makes the mix workable, sand gives it body, and fire clay is what makes it a high-temperature mortar.
How Fire Clay Is Used in High-Temperature Mortar
Fire clay is 1 part of every 6 in the mix. It's the heat-resistant ingredient that turns ordinary masonry mortar into high-temperature mortar.
The 3 : 1 : 1 : 1 Mix (by volume)
Measure by volume, not weight. Lime is fluffy and weighs far less per scoop than sand, so weighing the ingredients throws the recipe off.
The short version: dry-mix all four ingredients by volume until the color is even, then add water a little at a time until the mortar is firm and buttery, like thick peanut butter. Dampen your firebrick, lay thin, full joints, and let the mortar cure for about a week before your first small curing fire.
Our full guide covers every step, the tools you need, where to use the mortar (and where not to), and how it compares to factory-made refractory mortar.
Which Fire Clay Should I Buy?
Pottery suppliers sell clay by the temperature it's designed to be fired at. For firebrick mortar, choose mid-fire or high-fire clay.
| Type | Properties | Use it? |
|---|---|---|
| Low-fire clay | Very porous, not waterproof | ✗ No |
| Mid-fire clay | Excellent bonding, waterproof | ✓ Yes |
| High-fire clay | Excellent bonding, waterproof | ✓ Yes |
| Mortar clay / bricklayer's clay | Slightly different formula, fine for outdoor ovens | ✓ Yes |
What does "mesh" mean?
Mesh is the screen size the clay was sifted through. A 20-mesh clay passed through a screen with 20 openings per inch, and like sandpaper, a higher number means finer particles. For mortar, 20, 35 and 40 mesh are all fine. The only thing that matters is that it comes out of the bag as a powder, not chunks.
Fire clays builders have used
- Lincoln 60 Fire Clay, widely stocked by masonry and ceramic suppliers
- Hawthorn Bond Fire Clay (40 mesh), common at ceramic suppliers
- Plainsman Fireclay, an option for builders in Canada
- Brick mason's clay or mortar clay from your local masonry supply
These Are Not Fire Clay
- Premixed "fireclay" in a pail or bucket. That's refractory mortar. It can't be substituted for fire clay, and it won't blend with the sand, lime and Portland cement.
- Moist pottery clay sold in blocks. The recipe needs dry, powdered clay.
- Low-fire clay, backyard clay or garden soil.
Using mason's clay? Ignore the mix formula printed on the bag. Use the 3:1:1:1 high-temperature recipe above with the mason's clay as your fire clay.
Find These Materials in Your Area
Need it today? Check a store near you.
We ship every bag FREE to the contiguous United States. If you would rather pick it up today, all four ingredients are stocked at stores near you, so check these places first.
Call ahead and ask for each item by the names on this page. Counter staff sometimes suggest a premixed substitute that doesn't belong in a wood-fired oven.
Fire Clay FAQ
Where can I buy fire clay?
Start with a landscape & masonry supply store, which usually stocks fire clay along with the other three mortar ingredients. If they don't have it, a ceramic or pottery supply can almost always order a bag. Big-box home improvement stores carry it only occasionally.
How much fire clay is in high-temperature mortar?
One part out of six. The recipe is 3 parts silica sand, 1 part hydrated lime, 1 part fire clay and 1 part Portland cement, measured by volume.
Can I use mortar clay or brick mason's clay instead of fire clay?
Yes. Mortar clay and bricklayer's clay have a slightly different formula than pure fire clay, but they work well in outdoor brick ovens. Use the 3:1:1:1 recipe, not the formula printed on the bag.
Is premixed fireclay the same as fire clay?
No. Premixed "fireclay" sold in a pail is refractory mortar. It isn't the same material, it can't be used in place of fire clay, and it won't mix with the other ingredients.
What mesh fire clay do I need?
Any common size works, including 20, 35 and 40 mesh. Make sure it's a dry powder, not lumps.
Can I make mortar with just fire clay and sand?
We don't recommend it for a pizza oven. Without the Portland cement and lime, the mortar has no early set and no water resistance. Use all four ingredients.
Is fire clay the same as castable refractory cement?
No. Castable refractory is a separate product made to be poured into forms. Homemade high-temperature mortar is a mortar only and can't be used as a castable.
Please Order Carefully: Refractory Materials Are Non-Refundable Once Shipped
Refractory materials are heavy, costly to ship and can't be resold once they leave our warehouse. For that reason, refractory products can't be cancelled, returned or refunded once your order has shipped.
We're happy to help you get it right the first time. If you're not sure which product or how much you need, contact us before you order and we'll walk you through it.
Shipping
FREE shipping on every refractory product to all locations in the contiguous United States (the lower 48 states). Orders ship by ground carrier or commercial freight, depending on size, and freight carriers will call to schedule delivery. Please leave a phone number we can reach you at in the order notes.
Keep Building
Building a wood-fired pizza oven?
Our FREE plans include the full materials list, so you'll know exactly how much mortar to mix.
BrickWood has helped thousands of backyard builders.
Store availability and prices vary by region. Always follow the safety information printed on each bag.
