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Quartz Inversion


MatthewV

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As a material is heated up, it will expand slightly. The amount of expansion is different for different materials. This is more obvious with metals such as aluminum, copper and steel but still true of plastics, wood, and ceramics.

For the most part, the rate of expansion is linear; if going from 100 to 200° is a 0.001m/m change then going from 900 to 1000° will also be a 0.001m/m change.

 

The importance of Quartz Inversion is that around 573°C the rate of expansion changes slightly. Going from 100 to 200°C is a different fractional m/m from going from 900 to 1000°C.

 

 

--- ok, the non-technical analogy for potters ---

 

You are firing a large bowl. One side is represented by a red car and the other is a blue car. Both cars are driving down the road at 35 mph. Going at the same speed, they stay pretty close. But then the blue car starts accelerating to 55 mph! The distance increases (warping the bowl). A few minutes later the red car is also going 55 mph. In the end, the reach the same destination but the damage was already done.

 

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that is why putting a large circular piece into the center of a round-ish kiln helps prevent warping.  thanks for the technical reason, i just noticed that it worked that way.  can show you lots of sample warped bowls.

 

just loaded a kiln with 11 inch square plates in the center and an equal number of smaller ones surrounding it.  i hope it will even out the heat so the flat sides stay level.

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Great explanation. This is especially true for porcelain that has so much more silica/ feldspar in its content than any other clay. May I use this analogy in my classes? Potters often do not realize that the shelf will take much longer to catch up and thus keep anything touching it back. 

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At 573C   alpha quartz (trigonal) converts to beta quartz (hexagonal). The inversion temperature is the beginning of the "Phase change" for silica (quartz) also called flint. The changes in crystal structure lead to changes in the specific density: an increasing temperature corresponds to increasing vibrations of the atoms in the crystal lattice, and as these need more and more space, more open crystal structures are favored. At 573C, is when the vibrations of the atom reach their peak: as alpha becomes beta. Once that "earthquake" ends at 573C, the pieces can be heated at a higher rate.

 

Rule of thumb:   the larger the pieces, or more contact the foot ring makes with the shelf: the slower the temp climb during inversion. The temperature after can increase because the transition has taken place.

 

Nerd

 

Consolidation for future searches.

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Note that in the UK (and possibly the rest of Europe), quartz/silica and flint are not exactly the same. Quartz or silica refer to the same material as in the US, generally made from sand. Flint is, as its name suggests, made from stones of flint. They are calcined (to about 400C I believe) to make them weaker and then ground down. Generally flint melts a little bit earlier than purte silica, and also has a bit of calcium in it as the stones pick this up from the chalk soils they are generally found in.

As a matter of interest, but total irrelevance to here, the farmer across the road from us harvests flint, getting it out of fields both to sell and to improve the soil. He's just got 1000 tons out from another farm. This is mostly used for flint finishing to house walls, I suspect the share that goes to potters is pretty low.

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Sometimes I get a chuckle when thinking about this subject.  

Most of you guys are making smaller, functional work; the studio i work in is on the oppsite side of ceramics, lol.  Imagine trying to control this on a much bigger scale....like work that is several feet tall sculpture, 1-2" thick in some places and weighs maybe 300lbs...talk about the forces of nature/kiln firing showing its ugly face in some work.  Placement in a kiln stack has a HUGE effect on the work!

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Sometimes I get a chuckle when thinking about this subject.  

Most of you guys are making smaller, functional work; the studio i work in is on the oppsite side of ceramics, lol.  Imagine trying to control this on a much bigger scale....like work that is several feet tall sculpture, 1-2" thick in some places and weighs maybe 300lbs...talk about the forces of nature/kiln firing showing its ugly face in some work.  Placement in a kiln stack has a HUGE effect on the work!

...go on

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Yes would like to hear more on this. My own experience is that with larger forms most of the the problems we are discussing get magnified, and must be managed more carefully. Anything you can add is useful. Do you agree with the statements some others are making above about how to address this issue with larger forms?

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Well yes, the more mass in the form, the more they are susceptible to the forces of nature/physics/kiln firing/etc.  Hard to give specifics since it varies so much depending on what it is, how it was made, etc etc.

 

As for quartz inversion - 2% expansion doesn't seem like much to a little cup or plate, but to a 6ft tall sculpture with a 1" thick wall will definitely have some concern and you may have to make some big changes to accommodate making that item.  With bigger work it's inevitably going to be "moving more" as things shrink especially.  You actually have to engineer your sculpture to accommodate certain shapes, kiln firings, etc etc.

 

For example, everything made from clay will shrink to some degree and obviously clay can be heavy.  You may not be able to perceive the shrinkage from your average cup/mug, but if you scaled it up to a 24-36" diameter vessel you're definitely going to be able to see the shrinkage as it will be obvious at this scale.  Big heavy sculpture often gets vertical cracks originating from the foot, likely due to the heavy weight of the form and the friction on the work-surface as the piece dries, this continues as it fires and shrinks again.  Big work makes "walls" in the kiln that make heating uneven sometimes, so how you stack your kiln will likely effect the work too.  Example of that might be firing several long-necked forms - if you cannot place these centered in relation to the heat source, it's likely the uneven heating will warp the piece and it will lean toward the heat source since that side shrinks more.

 

If you fire very conservatively your chances of surviving these forces are much much higher.  Obviously you won't be able to fire big stuff on a fast kiln firing schedule, sometimes you even have to down-fire your work so it doesn't get dunting cracks. Keep in mind that my studio operates outside the box of your typical ceramics studio.  Most of our work is fired earthenware temp, with a forgiving stoneware clay body...oh yeah and did I mention most of our stuff is once-fired?!  Most of my gas kiln firings with big stuff takes about 1 week turnaround for firing - 1-3 day drying/pre-heat, up to 24hrs to fire, 2-3 days to cool.  Go slow from pre-heat to quart inversion.  Remember, it's not only quartz inversion to be concerned with...all ceramics still have chemical water that needs to come out too...so if you go too fast with a 1" thick piece of clay...boom....

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:blink:  :blink:  :o

 

i am so lost. i dont get it. i dont get the analogy.

 

what does the red and blue car mean? different materials or hot and cool part in the kiln? or just different sides of the bowl. so acceleration is rising heat right? why is one side heating more than the other? because of hot spots or cold spots or different materials? or is that not important? its more about warping happens because of different speeds. 

 

so then according to glazenerd's reply, in a bigger piece, the blue car actually starts cooling down? contracting? is that what reverse means? is it the end of the earthquake? no more extreme vibration?

 

i understand basic quartz inversion. i also kinda understand the expansion and contraction. the analogy throws me a bit. 

 

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silica expands when heated   the red car is expanding once direction, and the blue car the other direction. The heavier and thicker the piece; the more extreme the reaction is.  Look at the vibration of the silica atoms as seismic tremors: they start low and get stronger as the temperature rises until the earthquake happens. The earthquake metaphor simply implies when the atomic vibration hit their peak: once that occurs they stop shaking so much.

 

Nerd

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