Fire bricks versus 'Pressed Reds'

[david s]

Re: Fire bricks versus 'Pressed Reds'

Davids, How much do you buy your firebricks in Townsville?

I don't buy firebricks anymore. I sold the last of them that I had about a year ago. I use. A castable refractory to make ovens, it is way easier than cutting bricks. Most commercial furnaces use castable now, which is gunned on rather than using firebricks. When the refractory is past it's serviceability it is broken up with sledgehammers and a new lot gunned on. Most firebricks in Australia are now imported and hence their high cost.

[Volongo]

Re: Fire bricks versus 'Pressed Reds'

When the refractory is past it's serviceability it is broken up with sledgehammers and a new lot gunned on. Most firebricks in Australia are now imported and hence their high cost.

Ok. I was aware that you use castable refractory material for your oven, but just thought maybe you still use firebricks for the cooking floor. Using a sledgehammer to break the refractory material sounds like a good workout for me . Full body workout that!

The cheapest FB I?ve seen currently is $2.50 on special excluding delivery.

I have a question that you might be able to answer. What is the percentage make up of Metallic Oxides or specifically Iron Oxides in a typical clay pressed red here in Australia?

I understand that the chemical composition of one brick varies to next and from batch to batch, but I would love to see a technical data sheet detailing the average chemical composition of a clay pressed red?

[david s]

Re: Fire bricks versus 'Pressed Reds'

I have a question that you might be able to answer. What is the percentage make up of Metallic Oxides or specifically Iron Oxides in a typical clay pressed red here in Australia?

I understand that the chemical composition of one brick varies to next and from batch to batch, but I would love to see a technical data sheet detailing the average chemical composition of a clay pressed red?

Yes the amount of iron contained in any clay body will vary considerably. It is particularly important for firebricks as iron acts as a flux and makes the silica turn to glass at lower temps. But this is not really a factor for the temperatures we fire to, it only comes in at temps over 1200 C. Most Australian clays are high in iron, this may explain why firebricks are usually imported and so expensive here. Suitability for a WFO is not so dependent on iron oxide content, it is more the thermal shock characteristics and this will vary greatly for common pressed reds.

To answer your question look here

http://www.traditionaloven.com/artic...re-clay-bricks

[Volongo]

Re: Fire bricks versus 'Pressed Reds'

David, Thanks for the link, but I have read Rado's article on the firebricks, old red bricks as alternative etc.. The technical Data sheet is for a Fire Brick. I was curious maybe you have come across or know the technical data sheet for a red clay pressed brick.

I think you will agree with me that the content of a brick is very important as it determines its physical properties (strength). It is the strength properties of the material that determines its ability to withstand stress from expansion.

So yes, it is the thermal shock characteristics of the brick used that determines how long a WFO would last. By which Alumina has a higher strength (bonding) than silica or ferruginous clays. However, how significant the strength of alumina versus silica in relevance to WFO I am not entirely sure.

[david s]

Re: Fire bricks versus 'Pressed Reds'

David, Thanks for the link, but I have read Rado's article on the firebricks, old red bricks as alternative etc.. The technical Data sheet is for a Fire Brick. I was curious maybe you have come across or know the technical data sheet for a red clay pressed brick.

I think you will agree with me that the content of a brick is very important as it determines its physical properties (strength). It is the strength properties of the material that determines its ability to withstand stress from expansion.

So yes, it is the thermal shock characteristics of the brick used that determines how long a WFO would last. By which Alumina has a higher strength (bonding) than silica or ferruginous clays. However, how significant the strength of alumina versus silica in relevance to WFO I am not entirely sure.

Yes, i agree that the clay content is important, but no I don't agree with you that a fired clays strength determines its ability to withstand stress from expansion.
An example of this is a cob oven which not being fired at all has excellent thermal shock characteristics. Also the low fired pots made in South America, designed to be placed directly on a stove have good thermal shock characteristics, like wise raku clay pots which are removed directly from the kiln and dropped into water, have good thermal shock characteristics. Clays fired in the 1000 -1200 C (eathenware and stoneware) range don't have such good thermal shock characteristics, yet they are considerably stronger. Porcelain fired clays are good and are used for internal kiln bits and pieces like bung hole plugs and pyrometer probes which see big temperature changes, but at this temperature most clay bodies wil melt from the fluxes present. A brick manufacturer designs his bricks to be partly vitrified so they are partly waterproof yet can still breathe.Each batch of clay and each brick may have a different composition for house bricks because the clay is not highly processed. The manufacturer may not even be able to give you accurate details of his bricks composition. You may have bricks that will work ok or you may not. You'd have better chance with a creamy coloured brick than a red one.

[Volongo]

Re: Fire bricks versus 'Pressed Reds'

You'd have better chance with a creamy coloured brick than a red one.

David,

I did not write anything regarding the strength of a ?Fired Clay?. I was referring of the strength of the mineral content of the clay. That is the bonding strength at molecular level or simply molecular bonding strength. The latter is very important because by increasing it, the material becomes more thermal stress resistant. Of course there are also other factors that governs this such elasticity, coefficient of expansion, emissivity, homogeneity, thermal conductivity...

All relates to the content of the brick, hence why I am after the average mineral percentage composition of the red clay pressed bricks. But it?s all good a simple analysis with a handheld XRF is all that?s needed to reveal the element composition of the specimen.

<You'd have better chance with a creamy coloured brick than a red one.>
I am guessing you are saying this because obviously there?s a lot less Ferruginous clays in the creamy brick. Do a typical creamy brick contain more Alumina percentage wise than a red pressed one? Have you build oven(s) using the red clay pressed bricks?

[david s]

Re: Fire bricks versus 'Pressed Reds'

Ah, ok. I am not a refractory or chemical engineer. My knowledge of refractory materials stems from formal study relating to ceramics and practical observation over years of making ovens and kilns and firing pottery. Chemical composition of firebricks as quoted by manufacturers consistently quote the percentage of iron oxide present. Generally the higher the firebrick is rated the lower the iron oxide content. It's my understanding that the presence of iron is a problem at higher temperatures because it will act as a flux,reducing the melting point of the silica turning it to glass.. This is of little concern to us for a WFO because the temperatures we fire to get nowhere near this problem temperature. I do not know what effect iron oxide has in relation to thermal shock.
My kiln builders bible (Kilns by Daniel Rhodes) states that the good thermal shock characteristics of low fired clay bodies relates to the body being open i.e contains lots of grog, this is also true of raku clays and cob. In addition to this the lower fired clay body will have less shrinkage and less vitrification creating looser not tighter bonds. So this contradicts your theory of close bonds creating more thermal shock resistance via greater strength. I suspect for our temperature range iron oxide is not too important because the cob ovens and the south American pots contain plenty of iron oxide.(see my previous post).
Regarding the handheld XRF, I was not aware that they can read oxide percentages, I thought they only read the pure metals present. Maybe you need to ask a refractory engineer.
I have built an oven using pressed reds and yes it functions ok, but I've also seen lots of fireplaces in Victoria with pressed red brick floors that have badly spalled. I don't believe smashing a brick with a hammer is any kind of indicative test to its thermal shock characteristics, i think using pressed reds for an oven is like playing roulette some may be good others not so. If you find a better way of recognising this rather than trial and error please let us know.
IMO the main problem we have is that we tend to fire our ovens way too quick. 400 C /hr is way to fast for any refractory to cope with. The inside face gets so hot and expands well before the outside has time to catch up. Trouble is none of us have time to heat our ovens really slowly. Commercial ovens probably never really cool down enough for this rapid temp rise to be a problem.

[Tscarborough]

Re: Fire bricks versus 'Pressed Reds'

Any low fired pressed brick will work, some better than others. Extruded and high fired, cored or not, are not acceptable. I normally tell people to use the commons in the dome but spend the money for fire brick for the floor, both for durability and for the lower heat conductivity.

[Volongo]

Re: Fire bricks versus 'Pressed Reds'

David you are a wealth of knowledge and your contributions to this forum is greatly appreciated. Yes the Hand-held XRF is an elemental form of analysis. The oxides are not really important but rather the element in this case.

Below are some extracts taken from the International Journal of Applied Ceramic Technology—Brochen, P€otschke, and Aneziris Vol. 11, No. 2, 2014.

"Under operating conditions, important unsteady thermal gradients arise within components of a refractory lining. As the temperature increases/decreases, the volume elements of the components expand/constrict. Such an expansion/contraction generally cannot proceed freely in a continuous body, and thermal stresses are set up. Thermal shocks, that is, sudden transient temperature change, or process-linked cyclical temperature changes are typical conditions that give rise to thermal stresses in practice."


"The ability of refractories to withstand thermal stress is not a material-specific parameter, but rather depends on a large number of factors. These factors influencing the thermal stress resistance (TSR) can be divided into three groups:
1. Material properties, such as the coefficient of thermal expansion a, the thermal conductivity k, the thermal diffusivity a, the ultimate strength rc, and the specific fracture energy Gf
2. Component geometry, characterized by the wall/bricks thickness d of the lining
3.Heat transfer conditions, such as the heat transfer coefficient h, the temperature of the surrounding medium(s) (molten metal bath, hot or cold air, water, …), and thermal flux Ф"

"Thermal Stress Distribution: The thermal stresses in refractory bricks result from the temperature gradient that arises between the “hot face” and the outer cooler side of refractory linings. Basically, during heating, the free thermal expansion at the “hot face” is restrained by neighboring cooler zones and so stresses are produced."

"On cooling, the surface is subjected to tension, and the tensile strength of the material should be considered.
On heating, the surface stood in compression and fracture may take place due to shearing stresses at the surface."

"Basically, to avoid the thermal stress fracture (crack initiation), the maximum of the thermally induced stress distribution in a body shall not exceed its strength."

"In contrast to many technical ceramics and glass, refractory products are able to tolerate damage and, although fractured, retain enough strength to remain functional. In other words, the initiation of cracks in refractory components does not necessarily imply its catastrophic failure."

[david s]

Re: Fire bricks versus 'Pressed Reds'

The iron in a clay body exists as iron oxide, rather than as a pure metal. I'm not sure how the XRF reads this, but because the clay body in a pressed red brick is not particularly processed it is very inconsistent. There are lumps of stuff all through it and even a cursory inspection will show this. Variation from brick to brick and even different parts of the brick show this, which is what gives a brick wall an interesting variety of colour,tone and texture. I would presume taking a reading from any individual brick or different parts of a single brick will produce a wide range of readings. I think the material needs to be pulverized and ground to a fine powder to gain consistent readings.

[juliac121]

Re: Fire bricks versus 'Pressed Reds'

Red bricks can be used as a substitute for fire bricks. However, red bricks are not as hard and durable as fire bricks. Additionally, it does not conduct heat as well as fire brick and takes longer to reach adequate cooking temperatures.

[wotavidone]

Re: Fire bricks versus 'Pressed Reds'

Yes the Hand-held XRF is an elemental form of analysis. The oxides are not really important but rather the element in this case.

Handheld XRfs have disappointed me so far:
It's a technology that reads the surface, doesn't penetrate very far at all.
If you want a reasonably accurate reult on a rough brick, cut the brick with a wet saw, dry it thorughly and read the smooth cut face.
Or grind the dust exceedingly fine and press it in a cup with a mylar film over it and read through the mylar.

I think the speciation of an element, for example whether that Calcium present as the oxide, the carbonate, the sulphate, is actually very important. However, even in industry, we measure the elemental concentrations, and infer speciation from what we know about how the material was processed, and perhaps confirmatory XRD analysis.

Regarding the rest of your post, the literature quoted points to perhaps one of the most important things one can do to ensure longevity for an oven built from ordinary clay bricks - insulate it well and heat/cool slowly. This will minimise the temperature gradient, and thus reduce the internal stresses.

"Basically, to avoid the thermal stress fracture (crack initiation), the maximum of the thermally induced stress distribution in a body shall not exceed its strength." There's a statement of the blindingly obvious, if ever I saw one.

The other interesting thing about this thread is the reference to iron being a flux. Sort of. So are calcium, alumina and silica. It is the relative proportions of these that make the liquid phase less or more runny. The relationships between iron, silica, alumina, lime and zinc in the solid phase, rather than the absolute values, are probably more important when looking at the strength, and melting point, of a brick.