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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Dear Marc<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal>Does anyone know why folks recommend mixing sugar/molasses with clay when making rocket stoves?<br><br><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>There are two parts to the answer. The first is that any sugary solution will make the formed product stick together (before firing) and it is important with ceramics to prevent micro-cracks during drying because later that will be the place stresses accumulate and where the big cracks start. When ‘folk art ceramics’ are being made, they are usually so bad that anything you can do to improve the product will be acceptable. If adding sugar will keep the thing together long enough to get fired it is going to help.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>When you can choose what to put into the mix, you would add some bentonite for the same reason, but it would not get removed during the firing process. Bentonite turns liquid when mechanically disturbed and ‘freezes’ when that motion stops so it is good for unfired (‘green’) strength.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The second part is that during firing above 600 C (which most ceramics are) all the carbonaceous are removed (mostly by CO moving back and forth rapidly) and there is no carbon in the final product. That means there is literally nothing from the sugar (sucrose) in the final product so it has to have something to do with the pre-fired strength and drying process.</span><span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><br>* Jon and Flip mention it in the build instructions of a rocket stove on their <a href="http://www.rechoroket.com/Home.html">new site</a>. Some friends of mine just tried it in Cameroon, and it seems to work. They didn't have a supply of sugar, though. So they soaked + squeezed some bananas instead.<span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Bananas, quite by coincidence, have a lot of potassium in them as well as a high sugar content. Potassium, lithium and several other things act as a flux when heated in the presence of clay minerals, reducing the melting temperature so if the product was being under-fired before, it appears to have been improved by the sugar, but it was the flux. You could also try finely chopped fresh grass which has two fluxes in it.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Firing clay is not a single event. There are minerals that melt, one by one, as the temperature rises. At some point the blend of melted, sticky and unmelted ingredients has the characteristic you want. In a domestic stove, it is usually formability prior to firing, green strength and strong resistance to thermal shock. In some cases, increased resistance to heat conductance <i>R</i> is possible while retaining thermal shock resistance and strength and the ability to form, dry and fire the part. In nearly all cases, simple mixes of ordinary clay and sand are poor on all counts. Adding sugar can make a mix stronger and slow the drying before it is fired. A lot of microscopic damage is usually caused during drying of artisanal clay products but they are not usually put to the fire afterwards, so to speak, so no one notices. <o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Resistance to thermal shock dominates the demands of clay stove parts. Differential thermal expansion of the surface causes the clay parts to disintegrate even if never touched by fuel. Clay mixes that are traditionally made with trial-by-fire experimentation handed down for generations have two characteristics (that I had heard about) which make this possible.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The first is they have a low free silica content after firing, though it is quite by accident and the artisans have no idea what they have got in there. This causes the material to have a nearly linear expansion plot when fired, dramatically (not ‘significantly’) reducing the silica phase change (expansion from A to B phase) problem at 573 degrees. The addition of fluxes can contribute to this suppression, but by a different mechanism. It is possible to get very low deviations from a straight line expansion through 573</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>°</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> C without the product exhibiting a low </span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Ä</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> Length result. Technically it is not a low thermal expansion material but it appears to work like one in a limited temperature range. High free silica materials have a pronounced physical change in size at 573</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>°</span><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> – I can send you graphs.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Baldosa tiles are an example of this curious characteristic of low free silica which was discovered by Bruce Berger in a sample that actually contained more than 50% silica when analysed by XRD and XRF. The tile was from Guatemala and offered by Dean Still at ETHOS one year. It is an artisanal tile that works in stoves even though it has a relatively high thermal expansion. It does not work well because it is ‘porous’ or ‘insulative’ or ‘traditional’ or magical. It has a suppressed silica phase transition line. It cannot survive repeated high temperature changes for long but it is a lot better than pot-clay.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The winner is ceramic with a low thermal expansion. Stoves are much more likely to last longer if they do not crack in the first place. In order to get heat away from the surface of the combustion chamber and spread as evenly as possible through the material, it should have the twin features of high thermal conduction (which spread heat) and low thermal expansion (which reduces heat stress). You will notice that this heat conduction feature is directly contrary to the advice given by most stove construction advisors. That is why most homemade stoves ceramics fall apart as soon as they are heated a few times. It is hard to make industrial ceramics by accident and even if you do, can you repeat it?<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Adding lithium (often Petalite), particularly in the form of lithium carbonate, will greatly reduce the thermal expansion of ceramics if they are ‘set up’ in advance by tuning the mixture to receive it. Correct analysis of clay minerals can reasonably predict where the points on a phase transition chart have a workable mix. Working with Bruce for 3 years on this problem (2005-8) determined that having a low free silica and high strength was probably more important than ultra-low thermal expansion, though both approaches work. Flexible high temperature ceramics are another but unlikely possibility. They are expensive and finicky.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Obviously one way to overcome fracturing is to greatly increase the strength of the material. Adding sodium silicate to the clay in liquid form is a proven method. There are some Rocket stoves made in Uganda with the Ugandan flag colours painted around the outer metal shell. You may have seen them. The ceramic is very dense and hard and that hardness is from the introduction of sodium silicate and not adding any porosity. Porosity aggravates the cracking problem so the next time someone tells you to make a porous mix that simultaneously has a high thermal expansion and low strength, stop listening, keep moving. Industrial refractory materials feature a) low thermal expansion, b) mediocre ability to withstand large sudden temperature changes like those experienced by the walls of a stove, c) high working temperature (not usually needed in a stove) and cost-is-no-object. A domestic stove is one of the most demanding applications of ceramics. Cement kiln liners have a combination of high mechanical strength, low thermal expansion and high density, and high cost. They feel like stones. They make great kilns but you can’t cut them and are rare unless you live near a cement factory. In Dakar you can get them. “Refractory” means it can take high temperature, but it does not mean it can take high thermal shock. You can ruin a kiln just by opening the door a few times when it is 1000 degrees inside. Things fall apart. Fired materials have a cooling rate that should not be exceeded. One of the first questions a ceramicist will ask is whether or not the stove part will have water spilled on it occasionally. Rapid cooling is death to ceramics. Cold air admitted to the hot combustion chamber is similar death.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Porous ceramics: just a little more on this topic. One can imagine a flexible ceramic sponge that will survive repeated heating and cooling cycles through the silica transition temperature (573). OK, nice to have if you can find it. Hugh Allen, who made the Improved Kenyan Jiko Stove (IKJ as it is properly known) into a commercially viable product, says it takes a year of trial and error research to develop a rural, artisanal mix that will survive for a year in use as a charcoal stove liner. Note that is not a structural part, it is a part held together by steel sheeting. He wrote an excellent book on the subject in about 1991 outlining the artisanal approach.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The POCA stove (Maputo Ceramic Stove, MCS) uses the approach of limiting the thermal expansion (to prevent cracks starting) and high strength so as to provide structural components. It has no metal supports so is potentially a much cheaper product than a metal stove, with many added material advantages. The stove is produced by Peter Coughlin who subscribes to this list and he is probably getting nervous as I raise his product’s design features. He is bringing out a new product in a few weeks which features a blend that is a shift in approach from one of those above to another. This will bring down his cost and improve the product because he is taking care of multiple issues simultaneously. It is a technically advanced product though looks quite simple. The intelligence is applied early on resulting in a cheap, strong, durable stove body.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>* In rural Vietnam, a brick kiln engineer explained to me that they lined the inside of kilns with a traditional refractory material consisting of clay + rice hull ash + molasses.<span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>That is for the structural support prior to firing and limiting the shrinkage cracks. It has to (obviously) be fired very slowly in order to get the heat conducted evenly through the walls as they dry and de-carbonise.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>* While talking to some old colleagues, I heard that old moonshine stills in Georgia used to have a clay + sugar mix to seal pipe connections (very much word of mouth, could have been inaccurate)<span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Ditto. When fired, there is nothing left from the sugar which is organic. The mass lost is accounted for in the clay analysis report under ‘loss on ignition’ or LOI. Oil, decomposed wood etc is in the same category: organic material. It can increase the porosity and weaken the material if there is enough of it.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>* Apparently, in construction, you can retard the setting of concrete by pouring Coca Cola onto it while it's wet. Useful if a truck gets stuck in traffic when you're halfway through a pour.<span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>That is because the sugar (not the Coke <i>per se</i>) destroys the bond between the aggregate and the hydrating lime. Concrete trucks often carry a 50 pound bag of sugar in the cab for just the event you foresaw. There was a company in Canada that used to remove set concrete from large mixers if something broke down. They drilled thousands of tiny holes all over the rotating drum and used little explosive charges to break up the giant lump inside. My friend David Hadden asked the guy how often his services were needed and he said, “about once a year.” I guess that was enough. If a cement truck breaks down at the roadside, the driver is told to put the sugar into the mix. It prevents setting. If one put in coke, it would not delay setting, it would ruin the mix, guaranteed.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Does anyone know what effect the sugar actually has? Does it just retard the setting of the clay? <span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>It probably retards drying. Clay does not set. Elements within it melt and stick together (vitrification).<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Or does it help improve the material properties? (or does retarding its setting improve material properties when it's eventually fired?!?)<span style='color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>It retards drying in an environment where drying is poorly controlled. Because this sort of forces ‘proper drying’ as would be done by a good ceramics shop, it appears to be ‘doing something’. It is compensating for a lack of skill or facilities. At least partially. Proper milling/mixing first, Bentonite and controlled drying would do a better job.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>There is another approach that will also help which is to reduce the amount of water used in the mix. If it is not in the clay, you don’t have to remove it. Clay is hydrophilic so it usually expands when wetted. This approach was used by ProBEC in Malawi making very low cost ceramic top plates for single pot stoves and was discussed at the time at length on this list so the messages, drawings and photos are on file. The clay was dampened (if at all) with a little water then it was pressed in a manual device which formed the part with a very low water content. Wall tiles in bathrooms are made this way, with moisture as low as 4%, under very high pressure. This solves all sorts of problems. There is nearly no water in it to be removed so shrinkage cracks are zero. Storing green parts outside for a while is not likely to reduce the moisture content so if done properly, even at the artisanal level, they can by put directly from the forming process into the kiln and slowly cooked up to 120, held for a couple of hours, then heated to the final temp, ramping at perhaps 135 deg per hour to 600 and then 150 or more per hour to …..900, 1100 or whatever. If at any time the heating rate exceeds these values, it is likely spalling, fractures and cracks will appear on what could have been perfect products. The organics and water and crystal water have to be removed carefully.<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Incidentally regarding phosphates in the clay: phosphates act as a surfactant and aid densification with little pressure. You can turn a dry-ish mix liquid by adding phosphates. It is then possible to mix together large and small amounts of additives evenly and quickly for a homogeneous (accurate) mix. If someone put in phosphates accidentally they would also see product improvement without perhaps realising that they are compensating for poor preparation. You can re-stiffen the mix by adding a little HCl. Neat, huh?<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Regards<o:p></o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Crispin<o:p></o:p></span></p></div></body></html>