bed of silica

[Courtney]

How much silica sand  should I place between the kiln shelf and a large tile for bisque fire?  I am trying to prevent cracking.  Are there pros and cons for too much or too little? 

Thanks 

Courtney 

[Bill Kielb]

Smooth even distribution of grog or alumina is common. I think @Pres used to use a salt shaker for thin even application. Think of it as little ball bearings to roll over rather than a bed of sand so to speak. So a fairly small sparse amount evenly supporting the ware. No high spots, no large empty spaces in my experience.

[Courtney]

Thank you Bill this info helps.  In my mind I am thinking the the ball bearings analogy also allows air to circulate under the clay.  Is this poignant.  I know the air holds the heat differently than the shelf, so by having air passing through under the work, the work has a chance of having equal temp on top and bottom.  

 

[neilestrick]

It doesn't take much, just enough to keep it off the shelf. I prefer silica sand to grog because it's a smoother particle and works better as a ball bearing. Grog is angular and sharp.

[Bill Kielb]

5 hours ago, Courtney said:

 In my mind I am thinking the the ball bearings analogy also allows air to circulate under the clay.  Is this poignant.  I know the air holds the heat differently than the shelf, so by having air passing through under the work, the work has a chance of having equal temp on top and bottom.  

I like the ball beating thought. Truth be told air don’t do much in a kiln at 2000 degrees. I think the ability to contract without catching is super important. Almost all energy in a kiln after red heat is radiation then conduction. The air is simply too thin to hold any meaningful heat whatsoever. Most energy is by radiation, shelves warm by radiation then conduct to the ware through the shelf to foot interface, but the shelf often lags the ware temperature because of its mass and still the shelf heated and glowing is radiating the ware from underneath.

Too complicated for me, I think friction is a most reasonable explanation. Kilns to me in no way resemble convection ovens.

[Mark C.]

When the snow blows in the deep dark of winter all potters roll up in a nice deep bed of Silica to keep cozy

wait this is not about that at all!

 

 

Ok I mowed the lawn today so no snow here. 

[hlipper]

anyone have a recommendation on where to buy silica sand?

[Mark C.]

any good ceramics outlet  that carries glaze materials will carry it. If you are kiln building and just need some sand the lumber store carries lesser quaility sand

[hlipper]

13 minutes ago, Mark C. said:

any good ceramics outlet  that carries glaze materials will carry it. If you are kiln building and just need some sand the lumber store carries lesser quaility sand

thank you!

[kristinanoel]

Does anyone use a deeper bed of silica as a support for sculptural pieces, to prevent slumping?
So that the ware is kind of nested in a pile of silica, or in container? 

I've heard of this but not seen any images and not sure how it would work.
Can the silica be re-used? 

[Mark C.]

19 minutes ago, kristinanoel said:

Does anyone use a deeper bed of silica as a support for sculptural pieces, to prevent slumping?
So that the ware is kind of nested in a pile of silica, or in container? 

I've heard of this but not seen any images and not sure how it would work.
Can the silica be re-used? 

Yes it can be reused -keep it on the shelve and not in the elements. You can vacuum kiln after use if you get sloppy

most use it as a smooth flat bed not in a container.

[kristinanoel]

Thanks! 

The container - i'm thinking of that as a big low, flat tray, like an open top sagar, a sandbox, essentially, just to contain the silica for easier cleanup. 
Like a kiln shelf with a raised lip? 

 

[Callie Beller Diesel]

If you were to put the sand in a tray, it would run the risk of cracking like any waster or cookie I would think. Not bad for the clay piece, but might not save on a lot of the cleaning. But if the piece is of a size where it’s practical, just using a whole shelf instead of a half shelf would do the same thing. You’d have to have a wide enough container to tip the sand into, but that’s pretty doable.

[oldlady]

just saw this for the first time.  kristinanoel,  i have been using silica sand under my stuff for many years.  i use whole shelves and pour some, maybe a cup, not a mug full in the center.  this is not inside the kiln yet.   i then use a tool with teeth, like a tile setter would to spread the sand evenly across the kiln washed shelf.  i do not go as far as the edge of the shelf, maybe 3 inches inside the edge.  i have also used medium sized grog the same way.  the results are just a little different.

silica sand will eventually stick and stay where it is put.  but, because it is so lightweight, it tends to float in the air during the first firing and might land on something i have on a different shelf.  not good.  so i switched to medium grog.  maybe larger grog is better, i happened to have the medium at hand.  it does not move in the first or any firings afterward.  i am careful to stay at least 3 inches from the edge of the full sized shelf so that when i put the shelf into the kiln, grog does not roll off the edge onto whatever is below.  it tilts as it goes past the thermocouples but i control the tilt to keep the grog from rolling off.

since i make many large, flat pieces, i fire the grog laden shelves often without changing the grog,  sometimes it moves as i put things in and take them out of the kiln.  i refresh the area with a dry foam sponge sweeping in from the edges and run the toothed tool across the center again.    never inside or on the edge of the kiln, that is asking for stuff to be swept into the element grooves.

hope this helps.