Ceramics generally have better erosion, abrasion and corrosion resistance than metals. Alumina is the most widely used ceramic in tile form. Slightly harder, but more difficult to make, is silicon carbide. The biggest problem with tiles is anchoring them. If one tile comes loose, it removes a 1arge number of other tiles. Alumina also has problems with thermal shock. Attaching ceramics to steel has a lot of problems, mainly due to the fact that steel expands more than ceramic when heated. The best solution to this problem is a honeycomb shaped metal anchoring system called hexmeta1. It is welded to the steel backing and the ceramic is filled in the cavities in putty form. The other anchoring system is expanded metal, tacked onto the steel shell. Relatively thin ceramic plasters are used in this system, typically 15mm thick.
A refractory mineral, Al₂O₃.SiO₂, found in metamorphosed shales on the periphery of the Bushveld Igneous Complex. Andalusite grows in elongated crystals with a square cross section. Commercial grains are broken, not elongate. Some deposits are the variety Chiastolite, which displays a diagonal dark cross through the square cross section where impurities have entered the structure during growth. An alluvial deposit from Welverdiend yielded small elongated iron rich crystals which produced exceptionally strong abrasion resistant bricks. The “Krugerite” variety from Krugerspost is also iron rich. Most deposits do not exceed 3mm crystals, but some reach 12mm. Castables made from andalusite are common. Unfortunately most of them are “low cement castables” with a cheap matrix, and often disintegrate after casting, due to an unexplained reaction with the surface impurities. To test the quality of an andalusite castable, cast a sample and cut a wedge to see how fine an edge it can achieve.
For more information, see KYANITE.
ANGLE OF EXPOSURE
Radiant heat is transferred in a straight line, and the amount of heat transferred is proportional to what I call the “solid angle of exposure”.
We are all exposed to a heat source of millions of degrees C, the sun. The reason we are not instantly vaporized is because the solid angle of exposure is very small. We can burn our fingers by picking up a very small object that is hot, because the small solid angle of exposure does not allow us to sense the temperature.
We do not touch the hot wall of a large furnace because we can easily sense the heat due to the large solid angle of exposure.
Asbestos is a fibrous mineral. There are various different minerals, the commonest being Chrysotile. Crocidolite, or “Blue Asbestos” is a thinner fibre, less common but more valuable and occurring in longer strands. The dreaded disease Asbestosis is caused by asbestos being absorbed in the lungs. Unfortunately it takes decades to reveal itself, and after its discovery, thousands of people had already contracted it and died horrible deaths. To avoid asbestosis, asbestos must always be handled wet. It is now outlawed, and virtually unobtainable. It became very expensive, because suppliers paid huge insurance premiums to protect themselves, the insurance component of the products became several times greater than the manufacturing cost. Asbestos products were the forerunner of modern composite materials. Asbestos replacement products stretch the ingenuity of technologists. Keramicalia has been developing asbestos replacement products for over 25 years, but still cannot beat the original products. Asbestos products are harmless until you cut them dry or try to dismantle them. The only asbestos hazard today is from legislators insisting on their removal, thereby causing a totally unnecessary risk.
Steelworks melt iron ore with coke and fluxes in a blast furnace to form molten iron. Hot air is blasted into the bottom of the furnace from a ring of nozzles called tuyeres, pronounced “tweers”. You can make a small one in a paint drum. Wind a vacuum cleaner or pool cleaner hose around the inside bottom of the drum and lead it to the outside through a metal pipe. Plaster the inside of the drum with about 50mm of Keratuff 2, covering the vacuum cleaner hose and the floor. Now take a large diameter drill and drill holes through from the inside of the drum through the vacuum cleaner hose. Now make a small fire in the drum, and when it is burning well, add some charcoal and connect a vacuum cleaner in blowing mode to the end of your pipe. This will create an intense fire. Add a crucible with some metal into the combustion zone and it will melt quite rapidly. Make sure you have a long handle or tongs for your crucible and ensure that it stays in the combustion zone. Have fun!
CASTING LARGE PIPES;
The hydrostatic pressure is too high above 600mm to be contained by plastic or expanded polystyrene. The first layer must therefore set quite soon and the subsequent layers must be poured before the rapid advancing set and exotherm. The position of the set layer must be constantly monitored to ensure
The set does not overtake the pour.
The hydrostatic pressure of the fluid layer does not exceed 600mm.
The material is introduced through a pipe into the fluid layer. This prevents “splash marks” which a structural defects, and eliminates layering and discontinuities.
The geometry makes sealing of the bottom too difficult. Use spacers to centre the core and weights to prevent penetration under the mould.
Place angle irons on the mold corners and strap them tight.
Make a funnel attached to a tube the length of the pipe.
Make a stand to raise the funnel.
Make a graduated dipstick to monitor set level and top of fluid level.
Graduate the funnel stand.
Make a “blou-draad” poker to unblock the funnel.
Weigh out at least ⅓ of your mass in 2kg bowls for hand mixing. The critical factor to prevent joints is a short interval between successive pours.
Make a casting record to get data on setting rate.
Get a ladder long enough to reach the funnel at maximum extension.
Get a torch.
Get rags and washing buckets for dipstick, poker etc.
One man must be solely monitoring; recording mix times, temperatures, water %, set level and fluid level.
One man must be solely pouring and adjusting the funnel and pipe level.
Two men must be weighing and mixing.
Lessons from Green – Iron Tech pipe:
Number the bowls, otherwise you lose track.
Drag the dipstick against the less critical former.
The full length filling tube gets unwieldy and unstable. Change to a half length one at half way up.
Keep bottom of bowls clean, stuff can fall into mould. (It didn’t, but could have.)
Once the bottom has set, you can make 20kg machine mixes, as long as you have short interval 2kg mixes to get the material flowing.
The middle of the outside of the funnel sets hard!
If bubbles come out of the funnel, the tube tip has reached the set layer.
CHIMNEYS THROUGH ROOFS;
Several problems can be caused by chimneys passing through roofs. A hot chimney indoors creates hot air convection next to it and the hot air accumulates on the ceiling. This may cause ignition of wooden beams or charring. Insulation around the chimney is not ideal, because it causes the chimney to overheat and the heat cannot dissipate. The solution is to build a second pipe around the chimney, which allows the convected air to escape through the roof. This air also serves to cool the chimney. The gap between the chimney and the second pipe should be in the region of 50mm. The second pipe can stop immediately above the roof, preferably protruding just enough to stop rain entering it. A cap over the chimney or a skirt around it may be necessary to stop rain coming through. The downward extent of the second pipe determines the distance from which the ceiling receives radiant heat from the chimney. Insulation material such as ceramic fibre blanket or mineral wool or Insulag may be added to the outside of the second pipe, but this is not crucial.
“Cloning” is what we call making a ceramic copy of a component. Most often we get a broken ceramic part to replace.
We glue the pieces together with Pratley’s quickset epoxy, then fill in the missing bits with Plasticast.
We soak the repaired model in motor oil overnight.
We build a box around it, usually with a steel pipe section or bricks.
If there are holes through the model we place steel rods through them, protruding to the sides of the box.
We melt Multimould blue and pour it over the model.
Once it is cool, we take out the model. Sometimes we need to cut the mould to relieve undercuts or release enclosed holes. Then we pour in one of our Refractoramics, for example Electricast for electrical insulators. After about 2 hours we strip the casting and if necessary fettle it with our fettling sticks. Then we fire it, usually 1250°C with 5 hours soak. The finished product is about 1½ % linear smaller than the model. The process takes 2 to 3 working days from receipt of the broken model to delivery of the first fired components.
If no model is available, it can be machined 1½ % oversize out of any metal.
DUTCH OVEN; See Ovens
FORGE / CRUCIBLE MELTING FURNACE
Take a tin, about 25 litres. Make a hole in the bottom at a tangent and fit a hose around the inside of the bottom. The hose can be an old vacuum cleaner hose, Kreepy Krauly hose or garden hose.
Plaster over the bottom, the hose and the sides about 30mm thick, with Keratuff 2. Keratuff 2 is a powder, add water to plastering consistency. It comes in 10kg bags which will fill 12 ½ litres, at a 2015 price of R 220 + VAT for a 10kg bag. Leave it for 2 hours to set. Then drill holes, ±15mm through at an angle into the hose.
Attach a blower, vacuum cleaner in reverse or hair dryer to the protruding hose.
Make a fire in the bottom. When it is hot, add a bit of charcoal. After a minute, switch on the blower.
The charcoal heats up rapidly.
Place the crucible with a long wire handle onto the charcoal, add metal into it and pack charcoal around it.
It will melt aluminium easily, bronze or brass with difficulty.
The insulation of a furnace is usually composed of several layers. The inner layers of high temperature Refractories are generally less efficient than the lightweight outer layers.
Careful design ensures that the interface temperatures do not exceed the service limit of the outer layers.
If you add insulation to outside of a carefully designed furnace, you will push the interface layers to a higher to higher temperatures and damage or totally destroy the inner lining.
If you want to improve the thermal efficiency of an old furnace, add insulation to the hot face of the furnace. A 15mm layer of Fibre Plaster greatly improves the efficiency of a brick lined furnace. The Fibre Plaster has a low thermal mass, so it heats up quickly. It has low thermal conductivity so it reaches a higher surface temperature than brick and it protects the brick from thermal shock.
It is white and reflective, radiating heat back into the furnace. It is supplied wet in buckets as a ready to use plaster. It is very sticky and adheres well to refractory surfaces.
There are many variations, but most consist of a horizontal cylinder with a tangential hole for a gas burner. The casing can be any steel container, gas bottles are the most popular. There are 3 types of lining:
- Ceramic Fibre Blanket: High thermal efficiency but very short lived.
- Firebricks: Very inefficient and slow to heat up.
- Insulation castable: Robust and thermally efficient and fast heating. The most popular material is Hollocast 1, because of the great ease of installation and high quality and simplicity.
Place the cylindrical casing upright on a piece of cardboard. Find a tin or pipe smaller than your desired I.D. Wrap SFK cardboard (smooth one side, corrugated the other) around it to get the desired diameter. Insert a pipe tangentially through the casing into the SFK, to form the burner hole.
Calculate the volume of Hollocast 1 required:
(O.D. in mm ÷ 200)² X ∏ = area in dm² - (I.D. in mm ÷ 200)² X ∏ = area in dm² Cross section area in dm²
X height in mm ÷ 100 = volume in litres. X density of 1,5 = Kg of Hollocast 1 powder
Mix the Hollocast 1 powder with water, (about 4,4 litres/ 20Kg bag) until it starts to flow slowly. Pour it into the gap between the SFK cardboard and the casing. It will set and get warm in about an hour. You can strip the core as soon as the Hollocast gets warm. You can start heating immediately, with a small gas flame. If you hear steam hissing, turn down the flame, as you are approaching a steam explosion.
Borax drips and damages the refractory lining. The cheap and easy solution is to place a ceramic tile on the floor and replace it every time it is saturated.
A refractory mineral, Al₂O₃.SiO₂ . It is a metamorphic mineral formed under very high pressure. At different pressures and temperatures the polymorphs Andalusite and Sillimanite are formed. They all contain 72% Al₂O₃. On heating, they expand and decompose into Mullite, 2Al₂O₃.SiO₂ and Cristobalite, SiO₂. Kyanite expands by 7%, Andalusite about 3,5% and Sillimanite about 2%. Kyanite crystals are typically elongated, slightly flattened crystals, red or blue in colour. Its most common use is as a milled additive in the matrix to combat permanent linear shrinkage.
MAKING A COPY OF A REAL HAND;
- Dip hand in molten investment casting wax.
- Dip hand in ice water bucket.
- Dry hand.
- Repeat dip, dip, dry until a decent layer is built up.
- Wriggle your hand free. The fingernails are difficult to free.
- Place the wax shell in a bucket of sand.
- Mix equal weight parts of Reforce 90 and Reinite binder.
- Pour this into the wax shell*
- Leave overnight to set.
- Remove wax shell, either by chipping it, cold, or by melting the wax. Use a blowtorch or hair dryer to get the last skin of wax off.
- Cool and dry the model hand.
- Melt some Multimould Green in a microwave oven in a Pyrex or ceramic bowl.
- Pour it into a deeper container.
- Dip the model hand into it, then into ice water, dry it, dip it etc until a skin of 4mm is obtained.
- Allow the Multimould to cool to ambient temperature.
- Paint a thin coat of Multimould oil onto the skin.
- Strip it by turning it inside out.
- Turn it the right way round.
- Place the Multimould skin in a bucket of Hollofill.
*It helps to suspend a stick in it for later handling
- Mix Reforce 90 and Reinite Binder in 60:40 or 50:50 weight parts. Add pigment?
- Pour into the Multimould skin.
- Stand overnight, paint with Multimould oil and invert it.
Several other materials can be substituted for step 20. Some of these are:
Keratab Ultrafine: Brilliant white.
Kerapump 10: Brown with black streaks.
Zambezi Black: Pitch Black.
Plasticast: Easily colored, less brittle than above.
Marmould: Soft elastic fleshy feel, translucent.
Elasticast: Heavy and grey.
Keraset: Very hard, grey.
All the above should be on the moulding pricelist and data sheets on the website:
MICROWAVE OVEN; See Ovens
- Shovel on floor
- Pan Mixer
- Plantery Mixer
- Paste and let down
- Z blade
- Concrete Mixer
1. Shovel on floor
Use only for simple mixes with no ultrafines. Ingredients are usually not premixed. Pour ingredients on a heap. Mix with shovels. Form a crater in the middle. Add water into the crater. Mix from the inside outwards until everything is wet. Conventional (high cement) Refractories can survive this crude mixing, others cannot, they will end up with too high a water content.
2. Pan Mixer
This is standard for Refractories. Add the powder, start the motor and add pre-weighed water, according to the data sheet. The mix will usually look too dry after adding the water. Continue mixing until the material becomes workable. For low cement castables you may expect this to take 6 minutes. If you are forced to add more than 10% more than the suppliers spec, the material quality is suspect. Clean the mixer as soon as possible after mixing. Try to discharge the mixer completely after each batch, because material left behind has started to react with the water and will accelerate the setting of subsequent batches. Mixer blades wear and need adjustment to reduce the clearance from time to time. If the floor of the mixer is not visible behind a scraper, it needs adjustment. A rotary plough mixer does the same job as a pan mixer.
3. Planetary mixer, (Bakery mixer). This is the best type of mixer, and can handle all types of materials. The blade or paddle spins and rotates simultaneously and imports high shear to the mix. The speed can be varied and the bowl can be raised or lowered. For conventional mixing, add the powders, start mixing and slowly add the water.
For highly fluid mixes, a technique called “paste and let down” is used. Add about half of the liquid and then slowly add the powder. It forms a paste, in which all lumps are broken down. When the paste consistency is correct, the sides of the bowl will be cleared. Once you are sure there are no lumps, add the rest of the liquid, very slowly.
For high strength materials with a lot of silica fume, a technique called “puddling” is used. All the water is added first, then while mixing, half of the powder is added.
Once it is evenly wetted out, another ¼ of the powder is added and mixed until wet. Then 1/8 of the powder is added and mixed until wet, and the last bit of powder is added very slowly, keeping the mix wet.
If high silica fume mixes are not puddle, they may reach a very stiff consistency where they strain or break the mixer.
Dilatant mixes are also dangerous to mixers and can strain or break them.
4. Z blade mixers and ribbon blenders are similar to planetary mixers in their action, but the shaft is horizontal. This makes them ideal for lightweight mixes which tend to segregate in vertical shaft mixers.
5. Concrete mixers. These have a rolling action and impart low shear. They are unsuitable for most materials, yet they can handle dilatants materials better than other mixers.
Ovens are used for cooking food. I want to define them, since they are often wrongly named.
DUTCH OVEN; A heavy clay oven. Generally shaped like a horizontal half cylinder with a chimney at the back. A fire is made inside and allowed to get very hot for at least an hour. The fire is then taken out and bread is placed inside. The oven is then sealed and the bread bakes. Construction tip; make a good chimney. We have suitable designs and materials.
MICROWAVE OVEN; Food is directly heated throughout its volume by electromagnetic waves tuned to the vibration frequency of water. Heat can escape from the surface of the food, so the middle ends up hotter and the crust does not brown. A cockroach can walk across your food unharmed because his high ratio of surface to volume allows him to lose heat faster than the microwaves can heat him.
PIZZA OVEN; An oven in which the food cooks in ambient air. The cool, dry air passes over the food, then enters the wood fire , burns and the flame curls over the roof to be extracted just inside the top of the mouth. The base transfers heat stored in the floor by conduction and radiant heat comes from above from the flame. The moment you close the door you exclude the ambient air and the oven no longer functions as a pizza oven. It is possible to make a gas fired pizza oven, but I can’t think who would want one. I have a design for an electric pizza oven, but I have never seen one. A conventional closed electric oven is not a pizza oven, even if it is used for cooking pizzas. Pizzas cooked in static hot moist air will never taste right or be worth eating. Read more about pizza oven design, construction and materials in the precast section of our web site.
PRESSURE COOKER; Cooks in water/steam, no water or anything is lost. Temperature is above 100C. Normally heat is added very slowly, and cooking continues for many hours with no loss of moisture.
TANDOORI OVEN; An Indian oven consisting of a vertical cylinder of clay with a wood fire in the bottom. Bread or roti is cooked by slapping it onto the inside of the wall.
UMU; A Tahitian oven providing some of the best cuisine in the world. Very simple but lots of work. Dig a pit, build a big fire in it, add rocks to absorb the heat. Wrap your food in banana leaves, tightly, and place between the stones, covering it with hot stones. Cover the whole pit with soil or sand and compact it. Leave for a few hours to cook with no loss of moisture
In most applications the casting material shrinks very slightly on setting, e.g. the refractoramix range of pourable fire Refractories. (Electricast, Kerasic, Keratab Ultrafine, Keratab Fine Special, Justflow, Kerapump 10, Keralite 6 etc.)
The inner former therefore needs to be slightly compressible. For small ID’s latex or silicone tubing are good, usually with a steel rod through the centre to keep them straight.
I.D. formers can also be rolled up plastic sheet, x-ray plates are ideal. The centre of the roll must be filled with sand. These are used in a bucket of sand which keeps the I.D. and O.D. formers coaxial.
A third method of making I.D. formers is using PVC pipes. The pipe must be smaller than the desired I.D. It is cut in the length down one side. A spacer is inserted into this slit to push the pipe open to the desired diameter. Cover the spacer with adhesive tape. Place the rest of the pipe from which the insert was cut, inside the pipe to hold the insert in place. To strip the mould the spacer is removed and the pipe collapses to a smaller diameter.
For large diameters we generally find anything cylindrical (can be a stack of buckets) smaller than the desired I.D. Then we wrap SFK around it. (Cardboard flat on one side, corrugated on the other.) This is usually covered with a layer of thin polyethylene sheet. The assembly is stripped starting from the middle.
A fifth method is using expanded polystyrene. This has enough “give” to prevent cracking, leaves a rough surface but the surface is free of bubbles.
In many instances an outer layer of foam (“weather stripping”) is added to increase compressibility.
In practice, the method used is usually determined by what we find lying around.
Now for the O.D. formers. These are generally easier because no shrinkage needs to be allowed. A roll of PVC sheet is the easiest. It must have several layers otherwise it distorts.
With PVC pipes they are chosen larger than the O.D. and about 60mm longer. Slit down the length, leaving a gap ∏ x the desired reduction in diameter. Now the pipe is pulled closed, with jubilee clamps, releasable cable ties, or wires. Seal the inside with tape.
Often we use a drum larger than the O.D., reduce its I.D. with SFK and place PVC sheet as the last layer.
Centering the inner and outer formers is often done by placing them in a bucket of sand. A rope or electric cable of suitable diameter is very useful for centering. Sometimes we use weather stripping. Sometimes some masking is added for fine adjustment. These materials are placed between the I.D. and O.D. formers at the bottom.
To centre the inner and outer moulds at the top of the casting, three pieces of rope, cable or weather stripping (Foam plastic with sealed surface and adhesive backing, from Sondor in Sebenza. Some PVC extrusions and all polyethylene are slightly bent. To correct this, cut two lengths, slit them vertically on the same side, and fit one inside the other with the slit on opposite sides.) are pushed down the gap. At the end of pouring, these spacers are pulled up to just below the surface of the casting.
We never use steel in contact with a casting.
With all of these methods above the ends are rough, and the pipe moulds have to be made about 25 to 60mm longer, so that the pipes ends can be trimmed to the desired length.
If you start casting pipes often, you will develop an eye for cylinders of different diameters and start hoarding them in a store.
After reading this you are probably thinking “So what is the best?” I would say that expanded polystyrene, hot wire cut, is the best. A thin plastic liner gives a smooth surface, but not entirely bubble free.
The expanded polystyrene mould is designed as follows:
Start with a block of width 60mm greater than the pipe O.D. The length must be 40mm longer than the pipe length. Cut the I.D. and OD. With a hot wire machine. Then slice off a 20mm sheet from the bottom of the pipe. Remove the part the size and shape of the pipe + 20mm. Now take the 20mm ring from the bottom slice and place it between the inner and outer moulds. The cavity is now the exact length of the desired pipe. The outer mould has a slit where the hot wire entered to cut the I.D. This slit must be glued closed with silicone sealer. If you are not using plastic liners, put buff tape (duct tape) over the joint.
Our pourable refractoramics are very fluid, and the pressure at the bottom of the mould is high and easily underestimated. The pressure is proportional to the length of the pipe and the density of the material. If you cast a solid cylinder, the pressure at the bottom would be exactly the same as with a thin walled pipe.
To keep the mould closed, anything longer than 500mm, we put angle irons on the corners of the outer mould and strap them very tightly.
I think I need several photos to help explain things. I will start collecting photos of pipe moulds.
A group of materials made exclusively by Keramicalia. They fit in a gap between technical ceramics and refractories. They are fine powders, which when mixed with water become highly fluid and can be poured into moulds. The moulds are typically intricate rubber moulds made from Multimould.
Refractoramics typically set in one hour. They come in a variety of chemical compositions, including aluminosilicates, alumina, silicon carbide and zircon.
TO MAKE AN ELEMENT CAGE;
- Find a tin, pipe etc. slightly smaller than the desired I.D. (Crucible OD + 10mm)
- Wrap it with SFK one side corrugated cardboard, to the desired I.D.
- Wrap the SFK with buff tape.
- Cut strips of adhesive Sondor foam 10mm thick, 25mm wide, the length of the height of your cage.
- Stick the strips onto the buff tape with 15mm gaps between them.
6. Place double sided tape on top of the strips.
7. Cut Sonder foam strips 2mm wider and deeper than your element coil O.D.
8. Get a plastic sheet , fairly stiff, as wide as your element cage height, and 1.2 times the circumference
9. Lubricate the outer surface of the spiral formers and the inner surface of the plastic sheet with talcum powder.
10. Wrap a continuous spiral of these strips around the assembly, leaving a gap at the top and bottom, and 20mm between spiral turns.
11. Slide the plastic over the spiral formers to hold everything in place as you continue fitting the spiral.
12. Once the spiral former is in place with the plastic sheet over it, tighten the plastic sheet using jubilee clamps or wire, until the whole outside of the spiral formers is in contact with the plastic.
13.Get a bowl wider than the assembly and place some sand in the bottom.
14. Embed the assembly in the sand.
15. Add more sand around the sides.
16. Guess the weight of Kerasic required.
17. Weigh out 2kg and 1kg bowls, totaling twice your estimated mass.
18. Weigh out the mixing water at 170g per kg of Kerasic.
19. Mix the first 2kg bowl by hand, carry on for 1 minute after it looks evenly mixed.
20. Pour it into the top of the mould.
21. Add further mixes until the mould is full.
22.Tap the mould to release bubbles. The level should drop slightly.
23.Top it up.
24.Wait for the Kerasic to set and get warm. (1 to 3 hours)
25.Strip the assembly.
26. Fettle any material that got into the wrong place.
27. Fit the element.
28. Wrap with ceramic fibre blanket.
29. Cover with galvanized or stainless steel plate.
30. Make a Keratuff 2 floor.
31. Make a Keratuff 2 lid.
32.Wrap the end of the element tails with fine copper wire.
33. Connect with line taps.
Attaching tiles to hot surfaces:
The biggest problem is the difference in the rate of thermal expansion between the tile and the substrate. If you "glue" the whole surface there has to be release somewhere. It is better to use just a blob of adhesive in the middle of the tiles. You must also ensure that the tiles are ot touching at the edges. Leave a gap and do not grout it. The longer the dimnensions of the tiles, the great the problem. For heavy tiles, use Fibre Plaster as the adhesive.