Home-Brew Mortar what is the best ratio and WHY

[Mr.Pizza3]

I've been scouring the internet trying to find an answer on this subject and i cant seem to get any definitive forums discussing the two more popular ratios and their pros cons to doing either one.


With that being said it seems more budget friendly to create my own "refractory" mortar then it is to spend 70 dollars a bag when i need about 6 bags worth for the project.

Therefore I've seen two ratios thrown around for this "Home-brew" a 3-1-1-1 and a 6-1-1-1 also somewhere else i saw something like 10-3-1.5 or something which roughly translates back to 3-1-1-1

Anyway was wondering how many people have gone the 3-1-1-1 route or the 6-1-1-1 route and why you chose that ratio.

I've read that using refractory mortar is also not good because as you reach the top of the dome you have larger joints and this mix becomes too strong and can cause serious problems down the road to the integrity of your oven.

Also is the Home brew's insulation properties similair to refractory ? how much of a difference is there ? i know that the home brew is missing the alumna additive not sure how much that will play a roll


Finally is there such a thing as too fine a sand ?

[david s]

I've been scouring the internet trying to find an answer on this subject and i cant seem to get any definitive forums discussing the two more popular ratios and their pros cons to doing either one.


With that being said it seems more budget friendly to create my own "refractory" mortar then it is to spend 70 dollars a bag when i need about 6 bags worth for the project.

Therefore I've seen two ratios thrown around for this "Home-brew" a 3-1-1-1 and a 6-1-1-1 also somewhere else i saw something like 10-3-1.5 or something which roughly translates back to 3-1-1-1

Anyway was wondering how many people have gone the 3-1-1-1 route or the 6-1-1-1 route and why you chose that ratio.

I've read that using refractory mortar is also not good because as you reach the top of the dome you have larger joints and this mix becomes too strong and can cause serious problems down the road to the integrity of your oven.

Also is the Home brew's insulation properties similair to refractory ? how much of a difference is there ? i know that the home brew is missing the alumna additive not sure how much that will play a roll


Finally is there such a thing as too fine a sand ?

The 3:1:1:1 sand, Portland cement, hydrated lime and powdered clay (homebrew) has proved time and again a most suitable affordable and workable mix for wood fired ovens. It is not suitable as a mortar for kilns as their service temperatures are much higher. Because of the service temperature range we fire to, high temperature aggregates and cements are not required, so using them is a waste of money IMO.

Sand does not begin to melt until around 900C, in the presence of fluxes to help lower its melting point. I think any sand is ok to use but large grains can be a problem in mortar. I sieve my silica sand through a sieve, taking out anything bigger than 0.5mm if making up mortar.
Portland cement begins to degrade north of 300C so depending on its location will still be effective for the majority of the joints.
Lime begins to degrade north of 500C so will hold up for all joints.
Clay acts as a binder and as a super fine aggregate, but does not become permanent until it reaches its alpha change at 573C ( a process called sintering) and becomes harder the higher the temperature.
The two cementious components are the Portland and the lime.
The theory goes that as the temperature rises and the Portland fails, eventually turning into a fine aggregate, the lime takes over as the cementious material and as the temperature rises further the clay undergoes its chemical change, becoming the primary binder and the lime is left also as an aggregate
Most mortars work on a 4:1 cement to sand mix, as the joints should be weaker than the brick units they separate. The very high proportion of both lime and portland together in the homebrew would put cementious material in the mix at a ratio of 2:1 If we consider the clay as a fine aggregate. This may seem ridiculously high and probably why some folk have used 6:1:1:1, however the situation changes if the portland dies, leaving the ratio at 5:1.
My own experience with the mix has led me to reduce the clay content because although it imparts lots of stickiness which is helpful holding bricks in place, particularly at an angle, it has the unfortunate characteristic of increasing shrinkage markedly. Its addition to any mortar can increase the strength of the mix up to 7%.(by volume). More than 10% starts to produce shrinkage problems. The 3:1:1:1 ratio has a 16% clay content so I halve it (I COULD BE WRONG, but it works for me). As we don't get to 573C the clay will not turn permanent anyhow and the lime is still doing its job pretty well. It would be a mistake though to fire an oven too high in an effort to sinter the clay, because all kinds of other damaging effects can begin to take place in the 500-650C range.

The homebrew mortar is not an insulator, it is hard and dense with similar conductivity to that of firebrick.
Calcium aluminate cement can withstand much higher temperatures than Portland, but is an order of magnitude dearer, far trickier to work with and spoils far more easily.

[Mr.Pizza3]

so you do more like a 3-1-1-.5?

[Mr.Pizza3]

so as far as insulative properties are concerned refractory mortar is higher then the home brew but the home brew is going to withstand the larger gaps between bricks better?

[david s]

There are many different types of refractory mortars. Check the data sheet for thermal conductivity. The lower the conductivity, the higher the insulation value. Most have high density, high thermal conductivity and low insulation value.
For the temperature range we fire in I don’t think there’s any difference in performance between homebrew and any proprietary refractory mortar.
Because you want to spread the heat over the entire dome, I can’t see a highly insulative mortar being of any benefit. An insulating mortar would reduce conductivity between brick units as well as reducing strength. Having a similar conductivity to brick or cast refractory would be preferable IMO. Once the dense inner dome has been completed a highly insulative layer over it is required.

[david s]

so you do more like a 3-1-1-.5?

Yes, see post # 2

[Mr.Pizza3]

Okay would you recommend the home brew then over the high temp mortar from the store

[david s]

Yes, I would, as previously posted on grounds of cost, performance and workability I think the homebrew wins on all three. However, my answer would be different if it were a kiln. I’ve also said I could be wrong. There will be other opinions I’m sure, particularly from sales staff who want to sell you their product.

[JRPizza]

Mr.Pizza3, I don't have anywhere near the experience as david s does and most of us here defer to his advice on oven materials. I used the 3-1-1-1 mix with the recommended amount of fire clay as I was just following the recipe and as it was I ended up with a half bag of clay left over. When I build my next oven I would be tempted to try his recommendation (if I remember ).
I did want to ask where you got the notion that you wanted your mortar to have insulation properties - was it something you read or heard from a sales person? From what I have picked up mortar has 3 functions - hold the bricks in place while you are building and keep them from falling into the oven in service, not impart any toxicity into the oven by outgassing or dropping nasty stuff into your food, and provide some thermal mass in the gaps between bricks so the oven can hold more heat. Some folks even add extra mortar to the outside of the bricks to gain additional heat storage capacity. I can see zero reasons you would want to use an insulative material between dome bricks.
I have also read that the packaged refractory mortars do not tolerate gaps well, meaning they have a relatively thin bond line range that they provide the best adhesion in. If you were to use such a product you would probably want to maximize the tapering and beveling of your bricks to keep the bond line consistent with what the manufacturer recommends, but not because it is "too strong" with a large gap. The beauty of homebrew is you just need to cut your bricks to maximize inner face edge to edge contact (reduces exposure of mortar to oven temps) and use the mortar to mud in all the gaps on the outside.

[Mr.Pizza3]

Not sure, i may have misread about the alumna additive if that just increased its ability to reflect the heat back down as opposed to running without it. Not sure which forum it was talking about the differences of a home brew vs a store bought refractory

i also did read what your saying about the store bought needing to have the smaller gaps or it could cause cracking issues down the road. i plan on setting about a row a day so i will be taking the time to cut the brick for cleaner joints anyway

From your experience do you remember how many bags / weight you needed for the whole project?

I currently have 3 bags of 50lbs on the way for the clay but thats also to be used to set the oven floor bricks

i have a type S lime supplier with 50lb bags as well for that portion

i just want to get close to the counts so i can have everything waiting for me

[JRPizza]

I think you are overbuying. I used several bags of sand but ended up with a half bag of lime, portland, and clay. I might have used 20 lbs of clay max. You should read up on the cutting too. You can bevel the bricks as you go up to minimize the exposed Vee and just fill up the back gaps with mortar. Functionally there is little benefit to making the brick fit tight all the way around (in my opinion)

[Mr.Pizza3]

I know I’ll need the additional clay for the oven floor to set those bricks so I chose to go with a sole bag for that purpose and with pricing at 30 dollars a bag I figured it would hurt to have an extra bag kicking around just incase

[Mr.Pizza3]

Also did you have any specific forums you can point me towards as far as cuts go I do wanna keep my joints tight more for aesthetics

[JRPizza]

If you do a search for "taper AND bevel fornobravo" using google you can get a bunch of hits, and if you find one that says something like "more from site fornobravo" you can zero in on threads from this forum. Be prepared for lots of different opinions - your job is to separate the wheat from the chaff.
I just beveled the side of my bricks (you should have a look at my thread to see one of the many ways to cut the bricks). It was relatively fast and provides the minimum exposure of mortar to fire. This method uses more mortar than if you cut your bricks to have a uniform gap all the way around, but think about this - if you cut your bricks so there is essentially no gap, then when you mortar the bricks together there will be more mortar exposed to the inside of the dome (uniform mortar layer). If you tilt these perfectly cut bricks to eliminate the gap on the inside, you will not have net contact anymore and you might as well have just beveled. If you do what I did and many others have done, then you will have essentially line contact from brick to brick on the ID and the mortar fills the gaps to the rear, holds the bricks in place, and is predominately towards the OD away from the fire. Does this make sense or did I manage to mangle the explanation?

In the attached pics you can see what a beveled brick looks like and how this method gives you tight interior joints and forces the mortar and gaps to the OD