jrgpots Posted January 13, 2014 Report Share Posted January 13, 2014 I have a big bag of cristobalite. I have been reading that this forms in glaze or clay body where there is an excess of Silica. It is normally not a good biproduct. I can't find more about it. Please teach me..... 1. Can I convert it to SiO2? 2. It has a low inversion point of 200c. Can I use this to my advantage? Jed Link to comment Share on other sites More sharing options...
Biglou13 Posted January 13, 2014 Report Share Posted January 13, 2014 If memory serves me right it is. SiO2. Chemically same structurally different. I've read about it a few times. Causing headache...... Silca conversion, alpha beta, slowing temperature at conversion temp, most of what I've read and John b. Says too much or latent formation is a bad thing....... My reading understanding on subject matter is limited so please don't take my word for if. I'm interested hearing answer also. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted January 13, 2014 Report Share Posted January 13, 2014 Chemically it is SiO2. The crystalline arrangement of the silicon and oxygen atoms in the structure is what is different. As a RAW material it is often used as an additive to EARTHENWARE clay bodies to deliberately cause the contraction as the body cools in the kiln. This helps to put (some) earthenware glazes into more compression in order to fix the inherent tendency of these types of glazes to craze on the body. So with the deliberate addition of a high COE material into the body, the COE of the body can be made to match the COE of the glazes. Cristabolite will not form in a normal earthenware body because the tiop firing temperature is not enough to cause the conversion of any free silica in the body into cristabolite. As a raw material in a glaze melt..... it is simply supplying silica. If it were added to a high fire clay body, if there were enough fluxes present to cause tthe SiO2 to go into the melt........ it would not longer be crystalline cristabolite.... it would ba amorphous silica glass... and no issues of COE from that form would be present. It is a potential problem when it develops in a high fire clay body from an excess of silica.... which remains unfluxed and unmelted into glass, and then affects the COE of the body on the cooling cycle after it converts. best, .................john Link to comment Share on other sites More sharing options...
jrgpots Posted January 13, 2014 Author Report Share Posted January 13, 2014 Norm, I kind of inherited the bag from my grandfather the rockhounder who got it from a mine in Lompoc, CA. He had over 10 tons of rocks and stuff like this in his back yard when he died. The bag was in an old truck he gave me. It was used for weight over the tires. So.... I can use it like regular SiO2 in glazes. I can use it to change the COE of a clay, contracting the clay body to better fit a glaze or prevent crazing. Do I got it? Jed Link to comment Share on other sites More sharing options...
jrgpots Posted January 13, 2014 Author Report Share Posted January 13, 2014 My grandfather's hobby has influenced me. I remember driving down a road in the middle of nowhere when my grandfather cried out to stop. We stopped. He jumped out and started hammering at some rhyolithe at the edge of the road. When we got home we broke up the stuff to find Apache tears, drops of obsidian. Now I tend to look for rocks and clay also. Funny how people influence us. Jed Link to comment Share on other sites More sharing options...
Babs Posted January 13, 2014 Report Share Posted January 13, 2014 He wouldn't have seen that if he was focussed on his ipad! Link to comment Share on other sites More sharing options...
Guest JBaymore Posted January 13, 2014 Report Share Posted January 13, 2014 EARTHENWARE clay body, jed. Don't put it in a stoneware...... uinless you are sure you have a lot of flux available to convert it into amorphous form. best, ...............john Link to comment Share on other sites More sharing options...
ayjay Posted January 13, 2014 Report Share Posted January 13, 2014 The bag was in an old truck he gave me. It was used for weight over the tires. Sounds to me like Gramps knew best. Link to comment Share on other sites More sharing options...
Bob Coyle Posted January 14, 2014 Report Share Posted January 14, 2014 Norm, I kind of inherited the bag from my grandfather the rockhounder who got it from a mine in Lompoc, CA. Are you sure it is not Diatomaceous earth? That is what Lompoc is known for. Link to comment Share on other sites More sharing options...
Bob Coyle Posted January 14, 2014 Report Share Posted January 14, 2014 I got to wondering whether diatomaceous earth was composed of cristobalite so I did a quick search. Here is a link to diatomaceous earth in Digitalfire. http://digitalfire.com/4sight/hazards/ceramic_hazard_diatomaceous_earth_toxicology_351.html?logout=yes They claim that it contains no critobalite. Other links also say diatom skeletons are composed mainly of amorphous silica, with small amounts of quartz and cristobalite. So if it is diatomaceous earth that you have, then there is not much cristobalite in it. Link to comment Share on other sites More sharing options...
jrgpots Posted January 15, 2014 Author Report Share Posted January 15, 2014 Thanks for the update. The mine in CA is a diatomaceous earth supplier. Link to comment Share on other sites More sharing options...
Bob Coyle Posted January 15, 2014 Report Share Posted January 15, 2014 Diatomaceous earth is pretty much impure SiO2 but it has a huge surface area. It is used as a filter media and an adsorbent. It would be interesting to make a glaze out of it as a silica substitute and see how it effected the viscosity of the glaze and the finished outcome compared to just 300 mesh silica. No doubt someone has done this at sometime, but I don't remember ever reading about it. Maybe John B. has?? Link to comment Share on other sites More sharing options...
PeterH Posted January 15, 2014 Report Share Posted January 15, 2014 AFAIK the iron content can vary considerably, and could be a show-stopper for everything except iron glazes. There is an old article on using diatomaceous earth in glazes: http://onlinelibrary.wiley.com/doi/10.1111/j.1151-2916.1951.tb13010.x/abstract ... in general diatomaceous earth was superior to other forms of silica, in particular giving the glaze a good surface over a longer firing range. To quote: Diatomaceous earth is a hydrated, amorphous,low-expansion type of silica formed by the decay of microscopic single-celled plants called diatoms. This substance contains approximately 85% silica, the remainderbeing principally alumina, alkalis, and water. Digitalfire give a similar but more comprehensive analysis, with 1% iron: http://digitalfire.com/4sight/material/diatomaceous_earth_268.html?logout=yes So it might be worth giving it a go, basically subbing it for silica. Maybe start with a clear glaze to see the effect of any contaminants. Regards, Peter Link to comment Share on other sites More sharing options...
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