best support medium when firing large pieces: silica sand, vs alumina oxide, vs zirconia vs grog

[s6x]

I'm firing large pieces and got a brand new L&L jupiter kiln.   I was advised to use grog between my sculptures and the kiln shelf to ease the friction during shrinking, so I bought a bag of "large grog".     Now I'm reading the kiln instructions and it says i must not use "silica sand" in the kiln as it will destroy the elements.   L&L recommends alumina oxide or zirconia oxide sand instead.   

I know grog contains "silica".  but not sure that means it's equally damaging as silica sand.  And maybe buying "large grog" was a mistake? would s"mall grog" be better?  If anyone has experience in this area, I would love your input on the best medium to use  

[Callie Beller Diesel]

Grog is fired ceramic that’s been ground into sand. So yeah, it contains silica, but it’s not the free silica in silica sand. I’ve used grog for setting platters on and it’s been fine, but your mileage may vary with a sculpture. 
 

I’m going to tag @neilestrick and ask what the manufacturer is specifically worried about, because I’m not sure how  carefully placed sand grains would affect kiln elements. It’s not volatile. Is it the potential dust kicked up while placing the sand?

[Min]

2 hours ago, s6x said:

I'm firing large pieces

How big and heavy are they? Another option might be to use coils under the pieces with or without a waster slab. I don't like cleaning out grog or silica sand from kiln shelves so avoid using those if possible.

[s6x]

Thank you!  They’re about  100lbs.  And about 20x20x 27 inches.  

[neilestrick]

11 hours ago, Callie Beller Diesel said:

I’m going to tag @neilestrick and ask what the manufacturer is specifically worried about, because I’m not sure how  carefully placed sand grains would affect kiln elements. It’s not volatile.

I'm working on getting an answer to this.

[Roberta12]

That is why I have never used silica sand in my kiln. Because the book said not to!  

[neilestrick]

The issue is if the silica gets onto the elements it can cause harm the protective aluminum oxide layer that builds up on the elements. This is assuming that the silica gets hot enough to bond with the alumina. The lowest-melting eutectic of alumina and silica is 2813F, so I'm not convinced that it's going to happen in a pottery kiln. It may stick to the elements, but I don't think it's going to melt. Plus I've seen elements that still function even when fully encased in melted low fire clay. However, this still falls into the category of DON'T GET STUFF ON YOUR ELEMENTS. So if you do use silica sand, don't spill it onto your elements, and  vacuum it out if you do.

[s6x]

9 hours ago, neilestrick said:

The issue is if the silica gets onto the elements it can cause harm the protective aluminum oxide layer that builds up on the elements. This is assuming that the silica gets hot enough to bond with the alumina. The lowest-melting eutectic of alumina and silica is 2813F, so I'm not convinced that it's going to happen in a pottery kiln. It may stick to the elements, but I don't think it's going to melt. Plus I've seen elements that still function even when fully encased in melted low fire clay. However, this still falls into the category of DON'T GET STUFF ON YOUR ELEMENTS. So if you do use silica sand, don't spill it onto your elements, and  vacuum it out if you do.

Thank you so much for that info.  I am trying to figure out what the best support medium is for heavy works.   I bought large grog and it sounds like even though it contains silica it is not the same as “silica sand”.  Although I guess it could bond to the  elements as well based on what you are describing?   So what’s the best alternative?   I have no experience with this and have no preferences so I can buy anything recommended.   

[Jeff Longtin]

If you google "silica sand vs sand" you get interesting results. It turns out some types of "silica sand" may also contain things like "particles including salt".

I wonder if the possibility of salt contamination would create sodium vapors? (Especially if you fire cone 10.)

It would seem all "sand" is not created equal?

note: I've used sand to support fragile pieces, on occasion, and saw no discernable effect on the elements. (I fire cone 6-7)

 

 

 

[Bill Kielb]

2 hours ago, s6x said:

So what’s the best alternative?   I have no experience with this and have no preferences so I can buy anything recommended.   

I have always used alumina as my go to. Never so much as to infill gaps but so the item being fired can slide freely without binding as it  moves ever so slightly in the firing. To that end it is more a light salt shaker approach (not a bed of materials) and the kiln is hepa vacuumed at the conclusion of the firing. It would seem L&L folks are  ok with alumina and zirconia. I guess I would want to be sure you are not seeking to apply it with any significant thickness though.