Forge: Final Refractory Research

Ok, so the experiment with the clay and perlite is finally finished. Perlite already contains a lot of alumina, so I tried just mixing clay with powdered perlite (I used the bottom of a metal cup to crush it; a better option would have been a ball mill). The first brick was even parts (by volume). It cracked into two pieces and had a lot of spalling. The second was 2 parts perlite to 1 of clay. It broke in two, with only a little spalling. The last was 3 parts perlite powder to 1 part clay. The surface melted slightly, but there was no cracking or spalling. (I believe the last brick was tested after I started using fuel pellets, so the temperature was probably considerably higher than with the first two.)

So, my plan was to line the next forge with the 3 to 1 mixture of perlite powder and clay. I was also going to add some non-powdered perlite, but only pieces between 1mm and 3mm. I ran into a problem with this. Powdering perlite takes a long time. Also, perlite is mostly air, which means that an 8 quart bag of perlite yields only about 2-3 cups of powder. So, I started looking around for alternative refractory mixtures again.

This time I found a mixture that looks really good. The recipe is 2 parts clay, 2 parts silica sand (beach sand is primarily silica; play sand at hardware stores is also mostly silica), 1.5 parts perlite, and 1.5 parts Portland cement. I used this mixture, but I added the 3-4 cups of powdered perlite to the mix, and I also increased the portion of regular perlite, to improve its insulating properties.

I have not tested this mixture with heat yet, but I have finished the forge and will be testing it once it is totally cured and dried.

Forge: Refractory Materials

So I think this is the final conclusion of my research on refractory materials. There are two major types of refractory. The first is standard firebrick that is commonly used in making things like clay ovens. This firebrick is generally quite dense and contains a lot of alumina (28% to 80%+). It conducts heat well, which works especially well for transferring heat to food which you are trying to cook.

The second major type of refractory is insulating firebrick. This also often contains a large amount of alumina, but is likely to contain more silica as well. Insulating firebrick is usually very light and contains a lot of air. The best example of this is the HRSI (High-temperature reusable surface insulation) tiles used on modern space shuttles. These tiles conduct heat so poorly that they can be held in hand, while white hot, without burning the skin (see the Wikipedia article Space Shuttle thermal protection system). These tiles contain only 10% silica and 90% air.

The type I am looking for is the insulating firebrick. First, it should be simple to mix good portions of alumina with the clay, to get a high alumina content, which should help prevent cracking. Since I do not have access to the silica fiber material used for HRSI tiles, my next best option is to add some perlite, specifically smaller pieces. Large pieces are likely to weaken the finished product too much, but if I mix in a moderate amount of perlite consisting solely of pieces between 2mm and 3mm, I should be able to add enough air to offer good insulating properties without weakening the fireclay significantly.

I think that the clay may actually have sufficient alumina to work fine if I were to add around 25% perlite to the clay (by volume). This time around, I would prefer to add some extra alumina anyhow, just to be safe, but I would guess that the 30% alumina is probably plenty for the forge. Once I have the forge working, I plan on trying different mixtures. I will make small balls or bricks (very small bricks) of the various different mixtures, then use the forge to heat them until they glow, then allow them to cool. I will repeat this procedure on each mixture, either until the pieces crack, or until I get tired of testing them and can conclude that the particular mixture is sound and unlikely to crack for some time.

My current bottle neck is producing alumina. I am still looking for a good source of scrap aluminum (since most "aluminum" cans produced currently are actually made of steel). Maybe I will have my parents save soda cans for me. For now, I am producing some alumina, but very slowly and in very small amounts.

Forge: Fireclay

Ok, so I have recently discovered that there is a terminology issue with my research. It goes like this: Many of the websites I have been finding have referred to refractory clay as "fire clay", but, as it turns out, the term is more often used to refer to clays that are fired to make pottery, etc. The site that I obtained my silica/alumina ratios was one that used the term to refer to pottery clays in general. So, normal pottery clays usually have between 24%-34% alumina and 50%-60% silica. The clay I bought was manufactured by Seattle Pottery Supply, so I emailed them and found that their Klamath Buff clay contains about 50% silica and 30% alumina.

To overcome this terminology issue, I began researching the contents of firebrick, since the term "firebrick" is used almost exclusively to mean refractory brick (although, in some cases it is called "insulating firebrick", while low temperature firebrick is just called "firebrick"). Refractory firebrick generally contains from 37% up to 90% alumina, with successively lower amounts of silica. The higher the alumina content, the higher the heat it can handle, but the softer it becomes. So, my current priority is to use electrolysis on aluminum cans to make alumina. I am planning on using 50%-70% alumina (this will be fun math, since the clay already contains 30% alumina).

A few days ago (around the time of my first Forge post), I took a 12v transformer I had laying around and a 5 gallon paint bucket. I attached the negative lead to a piece of stainless steel that I had used for this sort of thing in the past. The positive lead was attached to an aluminum can. Both were submersed in water (the copper leads were suspended a short distance above the water, to avoid contamination; copper will corrode in these conditions, if it is in contact with the water) and I turned the transformer on. For an electrolyte, I used baking soda (at 12v salt breaks down into chlorine gas and sodium; the sodium reacts with the water, contaminating it). The baking soda may be making the water more conductive than I want, so I will be looking into this when I have more time. Also, there is some evidence that the baking soda may be breaking down, so I will eventually have to do some more research on this.

Anyhow, after the first can had been mostly oxidized, I attached a Boyardee Ravioli can to the positive lead, assuming it to be made of aluminum. The water quickly turned a reddish brown. Evidently many modern "aluminum cans" are actually made of steel. I suspect large juice cans are also steel. So, next time I need iron oxide I know which cans to use (once the forge is finished I will be needing some of this for making thermite, which will yield iron and alumina when burned; I will probably write an article on this subject once I get there).

I will post again as things develop.