Amy Woodhouse

Glenn Herbert wrote:Not getting into the toxicity or lack thereof, I have a few questions and comments about your math and chemistry. First, did the manufacturer say the 25% is by volume or by weight, and is that aluminum or aluminum oxide? Those factors would significantly affect the amount of aluminum in the bricks. I think the relevant factor is how big a volume weighs a milligram, thus how much powder would need to be scraped off with each pizza to be hazardous. If a milligram of firebrick powder (1/4 aluminum or 1/4 aluminum oxide, whichever it is) is a decent pinch, I think we can agree that nowhere near that much comes off with each pizza, let alone gets into the food or air that a person consumes. Hard firebrick are quite abrasion resistant, soft firebrick are not (and thus are not suitable in any way for an oven floor). I don't have equipment to measure a milligram of firebrick powder, but I could weigh out 0.1 gram and see how big a pile/pinch it is.

I specifically asked that when talking to the manufacturer and they said it is not by weight but by volume. I also found out what a cup of aluminum oxide weighs in mg (I state this in my calculation) for my conversion to weight and thus milligrams. I also called the CDC and they said 1 mg a day is the amount you can consume in all of your daily activities and be fairly sure not to get sick. And then I determined how many milligrams of aluminum oxide are in small cob oven. To give you an example that everyone can visualize, 1 mg of water is 1/50th of a drop of water from your average dropper...hardly a decent pinch. The result is the result...everyone can make their own determination as to the risk.

JC, NIH.gov also has medical papers on its website which label Aluminum under heavy metals if you don't like Wiki. And no offense taken, I find it humorous.

Right Bill, What we are talking about is the aluminum dust that is inevitably scraped off the bricks during use and then ingested via food placed on said brick. This is not relevant for stoves or people that don't cook directly on their fire brick (which most people do).

Ok, here is some math..JC use you gray hair and check the numbers.

A typical fire brick is 4.5" x 2.5" x 9" which is 101.25 cubic inches.
If you use 26 bricks for a 36" x 36" cob oven you have 26 x 101.25 = 2,632.5 cubic inches of fire brick
Lets assume each brick is 25% aluminum by volume (which is what a manufacturer told me is the lowest % aluminum they make) so you have 2,632.5 x .25 = 658.125 cubic inches of aluminum
There are 14.4375 cubic inches in a cup so you have 658.125/ 14.4375 = 45.5 cups of aluminum
A cup of aluminum oxide weighs 934, 523.54 mg so you have 42,520,796.5 milligrams of Aluminum Oxide in your cob oven.
The CDC says as long as you don't ingest over 1 mg of aluminum per day (or 1/40,000,000th of your bricks) you should not have any adverse health affects.

So there you go, regardless of hair color, those are the facts so everyone can make an informed decision. Let me know if I made a math error anywhere and I will update the post to correct it.

Ah, my foundational understanding. From Wikipedia: "Beryllium and aluminium, although light metals, are sometimes counted as heavy metals in view of their toxicity" This was meant to be a discussion with dialog and there was no intent to offend any gray hairs.

JC, more info on how you construct your cob ovens without aluminum fire brick would be very helpful

Ken, no fear here...just having a discussion which is what these forums are for. Sorry if you think discussing the effects of consuming a heavy metal irrational. The decision is not use fire brick or don't build a cob oven....as JC pointed out there are alternatives for people that choose not to cook on aluminum fire bricks. Not dwelling on uncertainty...just trying to weigh the facts and not do something just because " that's the way we have always done it".

I appreciate the discussion guys

Troy, when does consumption of aluminum oxide cross over from safe to not safe according the studies you have read?

JC, people traditionally put food directly on the bricks in the oven right? So you are effectively eating off of it and consuming any aluminum particles scraped off the bricks and incorporated into your food....much more than a pot which was the point.
Now, one may say "I am cool with eating minute amounts of aluminum oxide and don't think it will hurt" and I am not saying it will....just trying to hear some opinions as I know there are many that have built them on here and have opinion on the matter.

Ken, the use of the lower aluminum % brick for cob ovens does not seem to be common knowledge in the world of cob oven building. Good point.

Thanks Troy,

Sure, I get the "everything can kill you argument". If you drink enough water it will kill you right? If we said that about everything we would worry about any pollution, heavy metals, pesticides or herbicides in our bodies.

Fire bricks are 50-75% Aluminum Oxide so that's not really a minute amount. In a wood stove I would have no worries as my concern is not the release through heat but rather the dust embedded in food from the scraping effect on the bottom of the oven. If your scraping off dust from a product that is 50% aluminum oxide your going to get fairly high levels in your body over time.

Yes, I would like to find an alternative, that's why I posted this discussion. I would bet that most that have built cob ovens are not even aware of the composition of fire bricks. Are the same people that would not dare use an aluminum pan ok with using an aluminum brick because it has a cob roof?

I posted this because it seems the first thing most permies say when building a cob oven is " go get some fire bricks". As far as I can tell from my research aluminum oxide (which is what fire bricks are made of) is pretty nasty stuff but I am willing to be led to a source that says otherwise.

Is there any concern about the aluminum oxide content in fire bricks and the use of them in the floor of a cob oven? If so, what have others used as the floor as an alternative?

Tim, not all of us have the luxury of living in a country that has hunted all its top predators to extinction. Don't let your fear of guns cause you to make such broad sweeping statements.

Native plums produce very well on the edges in partial shade but seem to just survive as you describe in complete shade so getting more sun is the most important thing. You can run over plums with a bush hog and they will throw up suckers and come right back so they are pretty tough. We have a fairly large plum tree in our woods that was knocked over by a larger tree that fell on it. The plum tree rooted where it was touching the ground. I would wait until they are dormant just to reduce stress and cut them at the base. Next year when the suckers come up you can pile dirt around them and the following winter you dig up the rooted suckers. You can also just cut them or bend them over and root them as Leila stated...rooting the cuttings just takes a little care so they don't dry out. Good Luck!