Post by Forsythe on Oct 3, 2021 15:53:27 GMT -8
We've already established that it can start as low as 800C, and be completed by 1100C. Refer to here: donkey32.proboards.com/post/36620/thread
Peter and his source also state the same.
from very fine quartz found in some clays, from finely ground silica powder and from molecular silica liberated during the formation of mullite from kaolin.
Yes, that's the central premise here, from the beginning of my first post:
For those of us using these ceramic products, we should all be aware that the products we’re putting into our stoves are physio-mechanically not the same as the material resulting from being fired at temperature
If feldspar is present in the body then any available molecular silica is taken up in the formation of silicates, and thus cristobalite does not form.
This is yet another statement you've taken out of its original context, and attempted to apply it in a wholly different situation where it is not only untrue, but the exact opposite of this is true.
I think the issue here is that you're failing to consider the most important variables which act as mechanisms in crystal formation. This was the same issue in our discussion re: cordierite.
This is *not* a simple stoichiometric chemistry equation (where you start with a reactant, add a "reagent," apply a single temperature, get a resultant product, and you're done.) It is a continual process that keeps changing as we use the ceramic fiber in the flame path of our rocketstoves, continually impregnating it with wood ash as we rapidly thermal-cycle it.
Those variables are things like:
1) rate of temperature climb
2) "soaking" [holding single temps for extended periods] at various temperature thresholds
3) rate of cooling
4) repeated thermal cycling to temperature threshold(s)
5) rapid thermal cycling through all of the temperature gradients without "soaking" (as our rocketstoves do)
6) ALL of the fluxes present in the original ceramic composition — which act more as "dopants" than "reagents"— and which react in completely different, often opposite ways, depending on their ratios and other fluxes present. Yes, in some kaolin porcelain compositions, potassium from feldspar can impede cristobalite formation — but only on the first firing at temp, and only when other oxides and impurities are not present. When additional metal oxides are present (like sodium, iron, titanium, magnesium, etc.) the cristobalite-suppressing effect is not only lost, *it can be utterly reversed* This is HIGHLY variable depending upon the fluxes' ratios to one another. Furthermore: When that kaolinic porcelain (or kaolinic superwool) is repeatedly reheated at temp, it will undergo DEVITRIFICATION. This devitrification of kaolinic ceramics can cause the mullite to decompose to cristobalite. (as cited below) These fluxes, as we've established earlier, also have an enormous impact in catalyzing the low-temperature formation of cristobalite.
7) ALL of the fluxes which continue to be added from the wood's ash content. This is an unpredictable variable you cannot accurately know beforehand or model for, because various plant species contain very different mineralogical profiles in their ash content — even varying within the same species grown in different soils. This ash/flux content variable is significant because more and more ash is accumulated in the ceramic fiber the more a rocketstove is burned... and none of the studies on ceramic fiber insulation account for that. (Even though they do show that cristobalite DOES form, *even when only heated indirectly without wood ash present or continually accumulating in it.)
Waterglass and other reagents will have the same effect as feldspar.
This, as a blanket statement, is utterly false — for the reasons I mentioned above — and is even stated otherwise at the very beginning of the paper you cited. ["Waterglass," after all, is sodium silicate]
Your source information literally states the exact opposite of your claim:
Decomposition of mullite to cristobalite in aluminosilicate ceramic fiber (even *without* the added effect of fluxing wood ash minerals, which can accelerate it): Note here that POTASSIUM is cited as the cause, and that potassium (as noted earlier) is the greatest residual component of woodash, although not the only component, and is the active fluxing component in feldspar:
Study showing the devitrification formation of cristobalite in various ceramic fiber compositions:
Of significance, the above paper showed that cristobalite formation can be rendered harmless if the crystals can remain large enough to stay adhered in the deteriorating ceramic wool — or remain large enough that they are not actively carcinogenic once detached and ingested/inhaled (although they would still present a risk for general silicosis.) This is why it is suggested to seal the ceramic wool's surface with colloidal silica. Silica *may* act as a flux to encourage cristobalite formation — but it would A) help the ceramic fiber surface hold the formed cristobalite intact, and B) inhibit the wood ash from penetrating deeply into the ceramic fiber structure and decomposing it from the inside-out.
And I'm sorry to dismantle your theory, here, Karl, but this is an issue of grave importance to rocketstove makers and users. It is a materially substantiated warning, issued by health officials and ceramics manufacturers, themselves. We must not be cavalier in dismissing this for our own health and safety, and we certainly must not misinform others by spreading dangerously false claims and erroneous misrepresentations which inaccurately claim this cristobalite hazard is not a real and present risk.