Silica Sand For Use Under Slabs

[eoteceramics]

Hi,

 

I have some large slab pieces approx. 12" x 12". In the past I have had problems when firing ie cracking and explosions. Would using silica sand help and how exactly do I use it?

 

Thanks in advance, Julia

[Babs]

Explosions are prob. due to moisture, dry on outside and still damp somewhere in the interior.

Yes the sand would help but Marcia here does it all the time , and I have done it with flattish bird table tops, fired them on an edge, stacked in the kiln against a brick or each other.

Think others have fashioned racks to hold them.

there are a lot of posts about this in these forums.

[glazenerd]

eote:

 

Tile and slab work is my primary focus: using porcelain clay to boot. I have fired up to 24 x 24 x 3/4" thick slabs successfully. One of my most popular tiles is 14 x 18 with a raised arch. There are some tricks to the trade of firing slabs/tiles.

 

1. As Babs pointed out, if possible fire them on edge unless there is some specific reason you cannot.

2. SLOW DOWN the ramp cycles: the larger the tile, the slower the cycle. Pending on the moisture of the clay: 2-4-6-8 hours of candling is not out of the question.

3. Common for me to stand tiles on edge in racks while drying to a bone dry state (day after leather hard state) . If left on a flat surface, tops and edges will dry, but underneath is still moist. Either that or flip them after 36-48 hours.

4. Fire to 140F, hold for 2-8 hours pending moisture. Ramp to 195, hold another 2 hours (kiln load) 160F an hour to 1000F. 100F an hour to 1100F (quartz inversion range) go slow or you will crack it during this crucial range. then 160F to 1880F. I fire them slightly above the usual bisq of 1800F. makes them slightly more stable and dense, but still plenty porous enough to accept glaze.

 

Firing flat during glaze firing requires some silica under them, to allow movement. On larger pieces I have also used alumina hydrate, takes less and allows more movement. However, be sure to vacuum kiln afterward.  Do not get too pushy with the temps during glaze firing either 250F and hour or so. Programming a cooling cycle does not hurt either. Not uncommon for me to program a 200F an hour from 1800 to 1000F: because you have a large mass with large amount of glaze, with a large amount of surface contact. There is enough tension between the glaze and the tile, shelf contact just adds to that problem.

 

Nerd

 

This starts out as a 14 x 18 piece.

[Marcia Selsor]

I lean much bigger slabs on edge against shelves or bricks. They sit on coils so there is no drag. They get heated evenly.

Marcia

[eoteceramics]

Hi,

 

Thanks for replies, I'll follow your advice. But do you not get warping if fired on their side? Marcia, what coils do you use? and can they lean at an angle against the bricks? Thanks again.

 

Julia

[perkolator]

12"x12" isn't that big of a slab.  dunno what firing schedule you're using but likely it's too fast.

 

slabs and similar work with large flat surface areas against a shelf (like slabs, and big platters/pots without a foot trimmed into it) tend to have two main issues with firing.

 

-One is that they are a big surface area, sitting flat against a shelf, thus the heat cannot easily penetrate to the bottom, center, of the clay.  Because it was cooler than everywhere else during the time you were candling your kiln, by the time you go to your next stage of firing, this cool section kinda rapidly heats up and thus you get blow-outs. Raising/elevating the piece will help tremendously. Also slow down your firing to compensate for lack of heat penetration.  Kinda the same concept if you were to stack a really tight, low height shelf in the bottom of your kiln - the heat can't penetrate very well.

 

-Second issue is more likely with larger work than the scale we're talking about, and the problem is usually cracking due to the friction and drag caused by the physical weight of the mass as it's trying to expand/contract in the kiln.  Something under the piece

 

 

Yes, silica sand can work if you need to sit it flat.  Grog will also do the same thing.  Putting a layer under your piece will act as ball bearings and facilitate the movement as the clay expands/contracts.  Since it's not a solid, it will also allow moisture and heat to pass through the interstices and make its way to/from the bottom.  Another alternative is raising the piece on pieces of broken kiln shelf or clay pucks.  Most of the time with big work, I use either the broken shelf bits or balls of kiln wadding (1/3 each: grog, silica, clay) - they do the same thing, but they have one main factor that makes me go toward them before loose sand/grog......Grog/sand can fall through the shelf crack/joint to the work below, the wadding stays put.  Another benefit to the wadding and sand is that they will self-level out which is great if your kiln shelves aren't perfectly flat, but your piece was built on a flat surface.

 

Yes, slabs fired at an angle on their side can definitely warp.  Depends on how thick/thin, how heavy, how big, the temp you're firing to, etc etc.

 

I lean stuff on soft brick all the time as long as there's no glaze.

[Marcia Selsor]

I fire them as straight up as possible. Somewhere I have a jpg but I am in my phone now.

[eoteceramics]

Hi,

 

Thanks for all replies Perkolator, Marcia, etc I think you've  just saved me a lot of time and hassle.

 

Julia