One question I still had about the best way to cut the firebricks. I was trying with an angle grinder but i wasnt able to make the bricks the perfect shape. The angles were not 45 degrees always and i had a quite small disc so i had to cut from two sides. I read that a wet saw is preferred but im not quite sure yet what type of machine i should rent or look for.
It's sometimes called a "Wet Tile Saw" or [wet] Stone and Masonry Saw. It's a circular saw blade mounted on a water-tray table
with splashproof guards on the electrical components and the blade is usually tipped along the cutting edge with diamond-grit (sometimes with tungsten carbide instead of diamond, but carbide blades wear out pretty quickly.) Diamond lasts a lot longer because Al2O3 (
alpha-alumina, AKA "corundum" aluminum oxide) is the 2nd hardest mineral in existence, at 9 on the Moh's hardness scale — preceded only by diamond at Moh's hardness of 10. ...Alpha-Alumina is what makes firebrick refractory in wood burning, and what makes porcelain tile so hard and durable.... so the saw blades need to be tipped with something as hard or harder than the brick & tile they're cutting.
The reason they're "wet" saws: Cutting such extremely hard material creates an absolutely enormous amount of localized friction heat at the blade-work interface, which would burn away both the bimetal blade alloy and the diamond grit. The solution: a low-flow stream of water is steadily pumped from a reservoir over the saw blade at the leading edge of the cut. The water not only cools the blade — it also reduces the friction which generates the heat in the first place — and it entrains the brick/stone/tile-dust in the water, preventing it from going airborne and getting into your lungs and/or into the motor housing of your power tools. The blades are usually fairly large (10-13 inch blade diameter is common in north America) for making single pass-though cuts of brick, facing stone, and floor tiles. (Avoid the gimmicky hand-held models made only for very thin earthenware tile... they're not meant for handling brick thicknesses.)
Most construction supply / home improvement stores will have some for rent by the day — or will know where you can rent one if they don't hire them out, themselves.
The durability issue hinges on 3 main factors between pre-fired brick vs castable:
1) The
Al2O3 content (Alumina-to-Silica ratio) of the brick or castable. Higher Al2O3 will increase refractoriness and durability. Most castable will be available in higher Al2O3 contents (60, 70, or 80+% Alumina) than most common firebrick (most often 32-40% Al2O3 for common high-fired dense brick, with 38% being the average alumina percent for most widely-available dense firebrick.)
2) The
relative density (particle density) achieved in the final brick or cast shape (For castable, this hinges upon the 3rd factor — Whereas brick has already been thoroughly fired and fully densified.) After the Alumina-to-Silica ratio, the refractory's
density is the second most important feature for avoiding the corrosion / erosion / cracking / slagging / spalling caused by vaporized wood ash minerals [which are fluxes that react with —and ultimately destroy— aluminosilicate refractories.]. A dense (or coated & sealed) refractory can resist flux penetration far better than one with high relative porosity.
Contrary to factor #1, pre-fired brick will almost always be more-dense than castable refractory, simply because most people do not have a kiln to achieve the roughly 3000ºF (1650ºC) temperature used to densify high-fired, high-duty firebrick (even though the brick is often of lower Al2O3 content, on average.) Higher-alumina-content bricks *are* available, but they're more expensive and usually a little harder to source. That's why many people attempt DIY refractory casting for the core...with varying degrees of success...due to factor 3.
3) Whether
the manufacturer's process for molding/setting/curing/drying/heat-treating the castable is *diligently* followed ...at every step. (IE: Whether you're able to precisely mix, vibrate, and set the castable refractory mold to ensure pre-fired green strength & avoid dry-cracking, and then slow-ramping heat treatment of the newly cast refractory shape —eventually reaching temperatures at or above the stove's operating temp— to achieve density, chemical (slag penetration) corrosion resistance, and load-bearing mechanical strength.) This is actually a lot harder than one would think, and even some seasoned, professional masonry heater builders prefer not to use castable because of its fickle nature (which requires exacting water measurement and water purity, narrow temperature ranges for green-setting, extended drying times, and then a long, tedious, heat-treatment firing schedule to densify the cast's particles into a cohesive, dense mass. It is certainly possible to do it well... but there are many ways to mess up the setting, curing, and firing. (It often requires babysitting the cast with a spray bottle or keeping it tightly wrapped in celophane to maintain hydration through setting — and to avoid overheating or over-cooling while the temperature-dependent chemical reactions proceed and bond... and then finally "baking" the cast at progressively higher firing temps, over the course of many hours.)
If you'd like to know more, there's a thread specifically about refractory durability with more info here:
donkey32.proboards.com/post/37509/threadMetal stoves don't burn as hot nor as cleanly as refractory-lined stoves, and will rust away before long, especially with water periodically thrown onto the superheated steel. The thermal shock to the outside of the cage (holding the basalt rocks in place) will cause weld cracking after a couple seasons of moderate but regular use.
There are, however, a TON of refractory brick designs for sauna stoves— wherein the clean combustion is contained within a refractory core, and the sauna rocks are held within a replaceable (usually stainless steel) liner that rests in a large, open-topped cavity
on the outside of the stove's firebrick enclosure. The brick is very effective at transferring heat through to the sauna rocks — and yet it acts as a buffer between the 1100-1400ºC rocketstove flame temps and your steel liner and sauna rocks. It's said that the quality of steam heat from a brick sauna stove is much better than steel, because steel tends to overheat, then overcool, and send the sauna through a rollercoaster of too-hot-but-then-to-too-cool cycles.
Google image search has a nearly endless variety of Russian-style brick sauna stove designs.
