Crack in new cob oven

Greetings--New to this post and I'm sure something close has been discussed but here's my story: As you can see I have a healthy crack, what you see is the worst of it--about 3/8" wide at the extreme. This was my first time baking, so I had a pretty hot fire. First 3" layer 2 weeks ago, 3" insulating layer the next day, 90 degree air temps throughout. First firing about 5 days later, small fire, no cracking. Then a little hotter firing a couple days later (7 days from building now), and crack begins. 2+ hour fire enough to get floor to about 400 deg and crack gets a little bigger ( using Kiko as guide, he talks about starting small fires almost immediatly so I thought I was OK, especially with the hot days here). My cob recipe, almost no clay in soil so I added "fire clay", about 1.5 parts to 3 parts soil; 1 part clay/soil mix to a little less than 2 parts sand (mason's sand). I used a healthy amount of cedar shavings in the insul layer--the kind you get for pet bedding.

So getting to the crux of my question, assuming my oven is salvageable, what's the best way to fill this crack? I think there are a few contributing factors to why it cracked: the cob is a little thin around my flue, the flue itself presents weakness in the cob not only 'cuz of the thin cob around the front of the flue but it also has "seams" or joints that will show shrinkage. Not sure I had the ideal clay/sand ratio, and I now see I was probably in too big of a hurry to get it dried out and ready for pizza.

I think it's done shrinking, other than whatever expansion/contraction I'll always get when firing. I think cob mortar is my best bet. but what bout a silicon based calk or other concrete or mortar crack filler?

Long winded but I think I answered most questions. Thanks in advance!

Bob

Nice looking oven!

Even if it does have a crack. Does it go all the way through and allow smoke to escape from the fissure? If so, it might be better to rebuild it. If not, you could patch the crack with some filler (not caulk, more like clay with lots of fiber in it) and recoat the whole thing with a finish coat that has more fiber in it. As long as you rough up the surface so that there is good adhesion with the finish coat, you may be able to salvage it.

Apparently, you didn't have enough fiber to add tensile strength to the cob mix when it dried. This has been a problem at least since the time that Pharaoh ordered the slaves to make bricks without straw, so don't feel too bad about it. I find that putting newspaper through the blender with just enough water to get it to shred makes a good fiber additive for mud that you want to set without cracking. How much fiber to how much mud? I think that is a trial and error type thing, as muds seems to vary.

John the Pollinator--Thanks for the comments. The crack doesn't appear to go all the way thru--no smoke out of fissure. I had planned on using straw for fiber in the inlsulating layer, but in the end went with the cedar shavings--now I don't remember why. I'll try to patch with a high fiber/Ewell Gibons-type mix. I forgot to mention that I plan on adding a finish plaster layer, and I will be sure to add straw.

Hi Bob,

If smoke is coming out of a crack (which doesn't seem to be the case here??) then rebuilding is usually best unless well skilled in "keying methods" for cobb repair. If not smoke than just patch as described below. Never use opc cements or any portland based materials for repairing or building these structures and lime should only be used as a plaster never as a "building material" in most (not all) applications.

For a repair like this"

1.) When the oven is completely cool (not expanded!!) Take compressed air and clean out well the fissure.

2.) Coming from the top down on the crack measure 100 mm and take a chisel or 1/4 angle grinder head and make a periradicular "stitch key" that will later be filled with a length of fiber (e.g. hemp, sisal, flax, etc.) This "stitch key" is about 30 to 50 mm deep in most applications, but can be deeper. It shouldn't go more than 1/3 the full thickness of the dome. Plaster over with original earth plasters. Do the same thing for the length of the crack approximately every 100 mm to you reach the end of the fissure. Each "stitch" should extend between 75 to 100 mm on either side of the crack. Remember that small micro fissures are common on natural plasters.

3.) Mix a similar formula of the original clay bodied plaster that the structure was built with, and add, horse or cow manure to the mix, and if possible some "rice soup" or "cactus mucilage" if available. 3 cups manure to approximately 5 gal bucket for patch mix. When checking a sample after mixing the "fiber" from the manure should be visible and well distributed. The water is what is augmented with the soup or mucilage.

4.) Dampen repair area and apply patching compound.

This is one of many "patch mixes" that I know to work well.

Regards,

j

Thanks for your comments and for sharing your experience Jay. Smoke does not come from the crack, so the inside layer must be intact. For the stick keys are you saying space them 100mm apart so it ends up looking like the stitches scar on my ankle? And what should I use as the key, sticks? I have a mortar mix ready to add water and plan to add straw to the mix--seems this would add adequate fiber.

Since the crack expands when the oven is fully heated and then shrinks back to what you see in the picture-about a fat 1/4" at the max, I think I'll patch after heating the oven fully. That way the cob will shrink around the patch rather than continuing to expand from the patch when heated. What do you think?

Hi Bob,

I didn't say this before...sorry...but that looks like a really nice little oven!!

For the stick keys are you saying space them 100mm apart so it ends up looking like the stitches scar on my ankle?

Yep, you got it. The "stitch key" is placed every 100 mm or so. Lay into this slot that you cut a length of fiber (eg. hemp, flax, sisal.) I have gone back and added more detail to my last post.

I have a mortar mix ready to add...

When I read this...I get really nervous...as it usually means an OPC or other modern concrete or mortar material. These should never be used and will only lead to further failures. Use the same clay mix as the oven was originally made with. There is no need to add any straw to this "patching cobb" as it has no way of "keying" into the original matrix...that is why we use the "stitch keys." Micro fiber from manure will be more than adequate, and if this isn't available, and one cares to, then take 10 mm cuttings with scissors/shears from the extra "stitch key" fiber you select.

I think I'll patch after heating the oven fully.

DO NOT PATCH HOT OR EVEN SLIGHTLY EXPANDED!!!


This can (and has!!) split old oven directly in half. Your logic may seem correct, but the patch...if added when the dome is expanded...will act like a wedge and as the dome cools create added pressures. This patching material will either extrude out of the crack and/or cause further ruptures in the dome shell matrix if place in it while the dome is expanded. Follow the directions above.

Please let me know if you need any more detail.

Regards,

j

P.S.

Ovens like this traditionally are not left exposed to the elements but protected by a small pavilion, and/or plastered with a good lime rendering (ie. plastering) material. Even a "fat" lime wash would be in good order and better than just the clay itself.

Jay--thanks for the kudos. Yes, I really like the oven, my first. I baked in to once--got it up to about 5-600 deg, roasted some veggies first, then 3 pizzas, followed by a loaf of bread when the temp had dropped to the 350-400 deg range, followed by a pans of muffins!

I used the term "mortar" but I meant the cob mixture. My soil is clay-poor so I add "fire clay" and sand. That's what I'll patch with + some straw cut up < 1.5". And I'm glad I asked about cool or hot--I will do it cool, probably today.

I'm waiting to fire it several times before the final lime-plaster layer. I live in eastern Washington where the summers are usually hot and dry (hotter and drier each year unfortunately!). I could build a roof of some sort over it if I have to but I'd rather have an outer layer that I don't have to worry about (rain and snow other times of year). and I don't think I have to be that concerned with breathability 'cuz I don't plan on baking too much bread, so I won't be using it as Kiko does (remove all embers, close door and bake with radiant heat and steam)--mostly pizza, with coals pushed to the back while balking. Do you have a recipe for my outer layer?

Bob

Hi Bob,

Great...cobb patch...I feel much better now!

Do you have a recipe for my outer layer?

It all depends on the type of lime you get. I would experiment first with some test panels as I will presume you haven't worked much with lime. I think you may like just a really "fat" (i.e. thick) lime wash, perhaps and just add a coat at the end of each season during some dry weather and perhaps another quick coat in the spring as well.

I like many different blends, and will tailor those to the characteristics of the lime itself that I have access to. Find your local vender and type and I will assist and/or make further recommendations, there are many sources.

Let me know if I can assist further...

Thanks Jay, I'll let you know how the patch turned out and also when I start looking for lime. For the finish plaster Kiko recommends 3-4 parts sand to 1 part lime--is that approx. what you use? No soil or clay? Fiber?

Hi Bob,

For the finish plaster Kiko recommends 3-4 parts sand to 1 part lime--is that approx. what you use? No soil or clay? Fiber?

I am not sure who "Kio" is (can you give me a link??)

I will state, from my perspective as a teacher and facilitator, I am always leary of any information given as a "blanket recommendations" for a recipe without really knowing the ingredients. This is a perfect example of the rampant amount of "expertise" being offered on the internet and even within books, without really knowing the background of the source information. Many of these "experts" have no background in architecture, historical preservation, or related fields that work with these many natural materials, and are only speaking from what they have "taught themselves" or "watch a few others do." This may be enough in some cases...it really isn't enough in most...in my view. Vet your info well!

I have used lime renders that are 4 parts lime and 4 parts sea shell, I have used pure limes with only a dash of clay, and the list of mixes goes on for pages. It all depends on the lime, the purpose and the additives/augmentations...Each are very different. Does "Kiko" even mention the difference between "smooth sand" and "cracked," and how this effects finishes, or that sand should be pre dampened for at least 24 hours in most applications. I could (others that are experts have) write 3 volume books on lime renders, plasters, paints and mortars...This materials understanding and history is that complex, and anyone giving good guidance should share that first before blanket recommendations of any form...

Soil can be part of some "lime renders" for ovens, and can just clay (usually as a pigmenting material though it will add "fat" to the render and make it flow nicely in many recipes.) Fiber too is part of some recipes as well...I like fiber, but I am a bit obsessive about finish and about "historical accuracy" as well in many of the projects I have done over the years. I should also state...by my standard...compared to who I know and am associated with...my "lime knowledge and skill" is only at the "amature or advanced apprentice level," at its very best in my view...

Find your lime source (don't buy any) and check back in, and I will offer further suggestions.

Regards,

j

Jay,The Kiko I refer to is the author of "Build your own earth oven" I have the 3rd edition, Kiko Denzer. I'm a novice at this and from my web searches this past winter this is the name and book most often used as a reference for cob ovens.

Thanks for the warning about web "experts", I know there are many out there in all fields. I will start checking for lime sources and get back to you.

Bob

Hi Bob,

I felt like "dolt" after asking, because when I looked up his name...I recognized it immediately. This is a great average tombe on the "basics" of the subject, and also illustrates part of what I was saying...

Kiko is a wonderful artist, he presents as a very skilled baker, and now is an author as well...on a subject that could be expanded and explained much better... However, this demonstrates that anyone can publish any thing...especially in the field of "natural building." Many of the books out there (not all of them mind you) on the subject of cobb and related natural building, are written by authors that not by trade, training or education builders themselves, or even having extensive background in it. This is the main reason I warn folks to read with very "open and questioning eyes" everything they read on this subject...and...ask questions. Anyone that claim "some knowledge" on subjects need to be able to freely and openly field questions folks may have on these many subjects.

Look forward to you finding some good lime source for your project.

Regards,

j

:-]

Jay and others--My patch job seems to be successful. The stitch technique worked great. As you can see from the pics I used a hardware modification, reasoning that with wider ends (wing nut/washer) it would give more resistance to pulling apart. The last 2 show the cracks after a full-on firing--just minor hair-line cracks now that will be hidden/strengthened by the final lime plaster layer.

I'm now about ready for that final plaster. I checked with my local stone and masonry place and they have 1 kind of lime, the type they sell to masons, no other distinction. So what do you suggest for a mix? Also I'd like to make it as weather-resistent as possible; I think I'm more concerned with resistance to rain than having it be breathable, since almost all of my use will be with a hot oven, like pizzas.

Bob

This one shows the hairline crack after a good firing...

Hi Bob,

"...I used a hardware modification, reasoning that with wider ends (wing nut/washer) it would give more resistance to pulling apart..."

I appreciate the logic...

However, I would not generally ever recommending to anyone that they place "iron" (even if galvanized) inside a clay matrix ever. Besides just the "rust jacking" potential over time, metal has a much different expansion ratio than does cob and it also has a very strong tendency to create an interstitial zone of "moisture condensation" that if (when?) freezes can facilitate further damage. The method I suggest earlier is well proven and historically found. If a "butter fly key" of wood is need for some heavily stressed areas, then I would recommend those, yet in this application I don't really see it as a necessity per se.

It will be interesting to see how these "metal stitches" or "keys" perform over several seasons of weathering and firing...and perhaps the above less desired effects won't take place?

I'm now about ready for that final plaster. I checked with my local stone and masonry place and they have 1 kind of lime, the type they sell to masons, no other distinction. So what do you suggest for a mix? Also I'd like to make it as weather-resistant as possible; I think I'm more concerned with resistance to rain than having it be breathable, since almost all of my use will be with a hot oven, like pizzas.

Without knowing the "type of lime" and its source, giving a mix ratio is very difficulty (impossible??) I would follow the bags or supplier's recommendations (minus opc cement if they suggest it) and make some test panels...to compare performance characteristics...

It can be in a broad range from 3 parts sand to 1 part lime to as extreme as 7 parts lime to 5 parts sand and 3 parts clay...There are so many differnet formulations and most have to do with...

Application surface type...

Purpose of application...

Type of lime...

I must stress again...whether a house or a pizza oven...with all natural finishes...permeability (breathability) is a critical factor!!! As soon as something is "attempted" at being made "water proof," then the process of "interstitial moisture buildup," takes place and soon after, exfoliation, decay, freeze thaw fatigue and many other issues begin...

Breathable weather resistance, good shedding ability, and weather "wear resistance" is the only primary goals to be concerned with...

Flax oil, rice soup, cactus juice additives, and things like animal manures can all aid in "weather proofness." The primary application is not for the "mucilaginous" effect than can occur, but rather the consistent fiber matrix contained in most "Ruminant" waste. Any enzymatic reactions that also takes place and the mucilage contained therein, is a positive byproduct as well, and does aid in giving a more plasticine characteristic to the clay medium which in turn offers weather proofness. Rice soups, and other grain starch additives/binders sometimes play a role as well in the many recipes, as does cacti and seaweeds. Fermentation reactions, as far as I know, have little to do with these reactions in general, but do perhaps in other areas play a more vital role, such as "aliz" and other clay based finishes, and outer renders.

Uh oh, I guess I'll be the experiment with metal stitches!

I'll buy the lime tomorrow , then see what the bag says about mixing. Kiko Denzir says a basic lime plaster is made of 3-4 parts sand to 1 part lime, and just enough water to make it workable. Seems clay would be a good addition. Maybe wheat paste? How do I make "rice soup"?
I think you're suggestion of test batches makes sense so that's what I'll do this week.

Made 4 pizzas last night--this oven is awesome--I love it!

Bob

Hey Bob...don't freak out Buddy...you only have two of them and I wrote what I did for "other readers" so they don't try this...I have seen it work (at least for a while) but metal is never better than more "sympathetic materials." I think you will be fine...and if not...great learning and teaching moment for others...

I'll buy the lime tomorrow , then see what the bag says about mixing. Kiko Denzir says a basic lime plaster is made of 3-4 parts sand to 1 part lime, and just enough water to make it workable. Seems clay would be a good addition. Maybe wheat paste? How do I make "rice soup"? I think you're suggestion of test batches makes sense so that's what I'll do this week.

3:1 to 5:2 is about the mean average range...I am glad you like the idea of "test panels." Wheat paste is viable...Clay is an ingredient that will replace part of the sand in some mixes. Rice soup is just as it sounds...A rice gruel (test mix this also) that becomes the "water" for the lime mix...This is an ancient formula and many versions and types of it...Think "Great Wall" of China...that is the "solidifier" in the rammed earth of much of it...along with "blood" urine, ash, animal manure, eggs...

Where here if you have more questions...Love that you are making pizza in it already...Aren't these things just wonderful? I can't believe that ever house in America doesn't have one?...Many use to...

Blessings,

j

I use donkey poo .. horse poo will doo too in the last 5 cm of the cob mix of my Rocket Mass Burners .. not one little crack whatsoever ..

Horse manure is renowned for making excellent plaster.  I've heard it explained as being because the straw fibers in the manure are very short.