Pdx 42" update

[Gulf]

Some products labeled as “paver sand” have polymer additives that may not be suitable for refractory mortar.

[david s]

I think any sand should be suitable, but a graded sand (ie a mix of different grain sizes is preferable, but if it contains grains that are large and problematic for thin mortar joints, then these should be sieved out.
For the Portland cement use General Purpose cement.
For the clay, any should be suitable, but it needs to be powdered so it will mix in evenly with the other dry ingredients before adding water. Avoid Bentonite clay because its extremely small particle size leads to excessive shrinkage. The cheapest should be Bricklayers Clay from builders supplies.
For the lime, use hydrated or hydraulic lime, not agricultural lime.

[Macrinehart]

Thanks David! I like having the detail to understand the reasoning! So 3:1:1:1 it is! Regarding my other question, are there material spec guidelines for the home brew? Am I good using my paver sand, or do I need to find some specialized ingredients? Cheers!

[david s]

Hey there pro builders, I have a friend who happens to be a ceramic arts specializing in wood fired ceramics and kiln building for wood fired ceramics. He offers a different opinion on the formulation of mortar. His ratio is 1:1 sand and fire clay. Also he advises having a couple different screen sizes in the sand, half 40, half 70. The reasons, no chemicals, longer dry time, easier to work with and he's also considering deconstruction and reusability of materials in the long range plan. I guess, also, if your an unskilled laborer like me, it may be easier to undo errors in the build process and fix them. (Not that I'm likely to make any mistakes of course, since I have the measure 20 times, cut once and take a coffee break work philosophy).

So I've read a lot of posts about the recommended 3:1:1:1 home brew formula, and the advice to work in small batches and get comfortable with dry times, which are faster. I wonder if anyone will take up a discussion on the whys of the recommended home brew vs. my friend's advice. (He didn't have a strong opinion, just a preference for not using cement and lime).

I recall he also talked about the inevitability of shrinking and expanding, and may have felt his approach was preferable in accounting for that movement. But he also expressed that he was not overly concerned with small voids or cracks. His main concern on that front was water penetration, not heat loss. He talked about having more care with cracks on the exterior stucco wall.

So, any takers in extolling the merits of one formula vs. The other? Or maybe just linking to some thread that already addresses this?

One other question I had - what material specs are recommended for the homebrew ingredients? My friend say use a 35 screen fireclay, and mix of 40 screen and 70 screen sand. I haven found many recommendations on specs for home brew. And I happen to have an extra bag of paver sand (no additives)...can I use that?

That's a lot, sorry for the ramble and looking forward to the discussion!

Cheers!

Mac

The requirements for a mortar to be used in a kiln are very different to those of an oven. Sand, especially in the presence of certain fluxes will melt at around 900C. Most kiln interiors will well exceed this temperature. For this reason grog (ground firebrick) rather than sand is a preferable material for kilns fired to stoneware temperatures. A mix of grain size enhances strength just as we use aggregate in concrete. The large spaces between the larger aggregate get filled with large grains of sand, the spaces between those get filled with finer sand and the spaces between them get filled with the cement paste. For mortar the same applies, but you don't want any large grains which could interfere with seating the brick units. The finer the sand the more workable will be the mortar.

Regarding the clay, small amounts make a mortar more workable, however anything exceeding around 15% can make the mix a bit too sticky to work with and also introduces excessive shrinkage leading to cracks. At 573C clay undergoes the Alpha change that sinters it and makes it permanent (won't return to mud). As the temperature increases the "fired" clay becomes harder and shrinkage increases further. For a kiln this process makes a 1:1 clay sand mortar permanent, but for an oven probably only the mortar facing the fire at the crown, on the inside will achieve this temperature.

The use of lime in the mortar gives it a greater tolerance for higher temperature. Portland begins to fail north of 300C and lime 500C. If that fails the clay takes over as the cementious material.


The homebrew mortar is a highly suitable (and cheap) mortar for home ovens. The lime content actually extends working time in conjunction with calcium silicate (Portland) cement. It would not be suitable for the higher temperatures experienced in kilns. What is commonly referred to as refractory mortar is expensive and has a very short working
time because it relies on calcium aluminate cement which uses a different chemical reaction.(never mix lime with calcium aluminate cement, it acts as an accelerant.)

To seal up the door of my kiln, I use a 50/50 mix of clay and sand.
Attached are two pics of a spy hole bung made of 50/50 clay and sand that shows the shank of the bung (fired to 1120C) while the knob, exposed to ambient temperature remains unfired.

[Macrinehart]

Hey there pro builders, I have a friend who happens to be a ceramic arts specializing in wood fired ceramics and kiln building for wood fired ceramics. He offers a different opinion on the formulation of mortar. His ratio is 1:1 sand and fire clay. Also he advises having a couple different screen sizes in the sand, half 40, half 70. The reasons, no chemicals, longer dry time, easier to work with and he's also considering deconstruction and reusability of materials in the long range plan. I guess, also, if your an unskilled laborer like me, it may be easier to undo errors in the build process and fix them. (Not that I'm likely to make any mistakes of course, since I have the measure 20 times, cut once and take a coffee break work philosophy).

So I've read a lot of posts about the recommended 3:1:1:1 home brew formula, and the advice to work in small batches and get comfortable with dry times, which are faster. I wonder if anyone will take up a discussion on the whys of the recommended home brew vs. my friend's advice. (He didn't have a strong opinion, just a preference for not using cement and lime).

I recall he also talked about the inevitability of shrinking and expanding, and may have felt his approach was preferable in accounting for that movement. But he also expressed that he was not overly concerned with small voids or cracks. His main concern on that front was water penetration, not heat loss. He talked about having more care with cracks on the exterior stucco wall.

So, any takers in extolling the merits of one formula vs. The other? Or maybe just linking to some thread that already addresses this?

One other question I had - what material specs are recommended for the homebrew ingredients? My friend say use a 35 screen fireclay, and mix of 40 screen and 70 screen sand. I haven found many recommendations on specs for home brew. And I happen to have an extra bag of paver sand (no additives)...can I use that?

That's a lot, sorry for the ramble and looking forward to the discussion!

Cheers!

Mac

[Giovanni Rossi]

Welcome back Mac. I was hoping you were just taking a break from building. Looking forward to your progress updates.

[Macrinehart]

Well guy and gals, it has been a long and momentious summer filled with too many trials and tribulations to recount...but I'm back and getting ready to resume work on the oven. So, when I left off, I had laid my oven floor and cut bricks for the first course of my dome. I also have my indispensible tool built - used the magnetic door stop as the fulcrum. But I haven't decided how to secure it to the floor yet.

I made a jig for the compound angle cuts, but it's not well engineered so I'm looking into a 3D printing option to make it a little more precise and robust. And JRPizza I will take you up on your offer of help with the tapper jig you used for your opening arch. I will follow-up with the taper angle I need.

It's good to be back! I am looking forward to finishing the build this winter!

[Macrinehart]

I do have my tapers figured. Thanks!

[JRPizza]

Still looking for my fixture file. In the meantime, do you have all your tapers figured out for the arches? The link below has a spread sheet that can help - check it out and let me know what you think.

https://community.fornobravo.com/forum/pizza-oven-design-and-installation/pompeii-oven-construction/397291-arch-taper-advice-with-hf-saw?p=397293#post397293

[JRPizza]

PS, if you have access to a printer I can send you the file I used. Could also modify it if you needed a different angle.

[JRPizza]

With a 10" saw you are going to have to make taper cuts in two passes coming from opposite sides. I discussed how I did this on the linked post below. It will help to pre-sort your bricks to get some that are square or at least have the top and bottom sides parallel. Because you are making cuts from both sides with hopefully a constant set angle, if the sides aren't parallel your cuts will be at different angles (does that make sense?). I also like you found I had to remove the splash guards to make a deeper cut.
https://community.fornobravo.com/for...post382412

[Hamilton]

Good morning.

I have not used a wet saw, my bricks were cut with a 12" double bevel compound miter saw so I am not able to provide a solution but I'm sure somebody will respond soon.
It is good to see you back it!

[Macrinehart]

Hi Hamilton - been a busy few weeks with travel, work and family, taxes, etc. I managed to get out and clean up the mess, no serious damage except for the total loss of the canopy.

I have hit a bit of a roadblock on the build. My issue is that I'm trying to cut tapered bricks for the opening arch, but the 10" saw does not have enough clearance to cut through 4.5" of material. Furthermore, the splash guards on the side and back of the blade housing are obstructing on the deeper cuts so I cannot push the brick all the way through.

I've considered removing the splash guards and flipping bricks to finish the cuts. It just seems like it's going to be tedious so I wonder if there is a more elegant solution?

The other challenge I've uncovered is that the jig I built needs some work. The minimum slope on is not shallow enough due to the heads on the carriage bolts I'm using. I think I may need to create a recess on the lower board so that the bolt heads can recess completely.

I'm also at the stage where I need to start thinking about mortar but I haven't started as I wanted to solve these other issues and so I can more forward efficiently when I start the dome.

Anyway - that's the latest from Portland.

[Hamilton]

Hello, anybody out there?
Have you thawed out yet?

[Hamilton]

Dad gummit! Mac.

Hate to see that!
Hope the saw survives! I'm sure the oven components will dry out and be okay.