Box Casting.Has anyone experienced devit?

Judi Charlson · Post by **Judi Charlson** » Tue Feb 24, 2004 10:15 pm

Joined: 10 Mar 2003
Posts: 46

Posted: Mon Feb 23, 2004 8:44 am Post subject: is there a dvit fix

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I originally posted this under kiln forming.No Response. Now under kiln casting, Has anyone casted in vermiculite boxes a la Bullyseye TIP 5 ?
I just took three box cast reverse relief pieces out of the kiln.
They turned out nicely, except there was some clouding and "pruning " around the edges of two. Plus a few"fins"on the edges.
I plan to just cold work the edges and clarify the rest with Watco oil or something.
I followed the directions from the WG conference workshop, except that I increased time to melt from 1 to 2 1/2 hours and increased annealing time from3 to 10 hrs, ( I used 2 1/2 billets instead of two because my reliefs were deeper.I hand another larger plaster cast in the kiln too)

My question: Was the melt too long?@ 1600.
Should it have been shorter and higher(1650).
Should I have crashed the kiln to lower temp to 960 faster?
If anyone has used this box method and has suggestions about devit, pruning and fins, please respond.
Thanks
Judi

Carol Craiglow · Post by **Carol Craiglow** » Tue Feb 24, 2004 11:13 pm

Hi Judy...I saw the Cauduro pieces that were box cast at BE last fall and noted that there was some hazing on the back of the glass...didn't notice any puckering.

Also, the hazing you got could be the result of firing plaster molds in the same firing cus sulfuric gas is emitted from the pl/sil mods when taken to casting temps and definitely causes some scumming on the back.

Carol

watershed · Post by **watershed** » Wed Feb 25, 2004 2:02 am

UYmm Carol, Sulphuric being released by Plaster silica molds? I can believe, but this is the first I've heard of it. Please explain.

Greg

charlie holden · Post by **charlie holden** » Wed Feb 25, 2004 11:09 am

I think the key to avoiding devit and spiking around the edges of box casts is to keep the edges of the billets well away from the sides of the molds. You want the glass to melt down into the center of the molds then flow out to the walls when it is very hot. You also want the binder to burn out of the fiber paper completely before the glass gets soft. Try to keep any kind of dust, like plaster dust, off the fiber paper.

I don't think the changes in temperature should make much difference unless your kiln cools very slowly. I don't crash my kiln when I do these, but I don't have my schedules in front of me. That being said, I've had occaisional problems with devit around the edges myself. I've about decided I will have to cold work the entire face of all of these pieces, if I decide to do a lot of them, just for the sake of consistant quality.

Email Ted Sawyer at Bullseye and see what he says.

ch

Judi Charlson · Post by **Judi Charlson** » Wed Feb 25, 2004 1:12 pm

Thanks Charlie,
I think I will email Ted. because I plan to do many more and many more sizes.
It would seem a shame to have to cold work the top after using expensiv e billets for the sake of clarity.
Also, at the workshop the edges were clear. Mine have texture of the smoother side of the fiber paper. They said that they had not cold worked
the examples.
Personally, I do not prefer the very smooth beveled look for my work which is mostly figurative and gestural looking.But the finish should look
somewhat planned and not flawed/
Judi

charlie holden · Post by **charlie holden** » Wed Feb 25, 2004 7:04 pm

Bullseye may be using a very smooth grade of fiber paper if they have clear edges with no cold working.

When I said that I may be cold working all of mine I didn't mention that I'm using cullet, sheet and pot drips. I can't afford billets yet.

ch

Carol Craiglow · Post by **Carol Craiglow** » Wed Feb 25, 2004 7:09 pm

Hey Greg...Yes, someone who's done lots of BE casting told me that last August, re the sulfur gas.

And I just read it again recently...have to track down the source....I believe its is SO3minus that is released from the CaSO4 in plaster.

Carol

Lauri Levanto · Post by **Lauri Levanto** » Thu Feb 26, 2004 4:59 am

My sources and experience is that
-plaster releases half of chrustal bound water between
42C and 240 C. In this range you can feel the moisture in kiln athmosphere.
This process is reversible. Left alone the plaster will
reabsorb the water from room air.
- The rest of chrystal bouns water is released somewhere
around 700-750 C. At this point plaster loses its strenth
which is tols in all books.
If you weight a dried mold before and after casting,
there is about 20 percen weight (water) loss.
SOmewhe, very much hotter, over 1000C the plaster
is totally decomposed leaving sulphur vapors
and grains of CaO. This does not happen in kiln
temperatures. The sulphur SMELLS.
-lauri

Lauri Levanto · Post by **Lauri Levanto** » Thu Feb 26, 2004 5:03 am

What is the difference between billets and cullets?

-lauri (making dictionary)

Judi Charlson · Post by **Judi Charlson** » Thu Feb 26, 2004 9:10 am

Hi Lauri,
Billets are something new from Bullseve. They are preformed transparenr
rectangles of glass 5" x 10" x 3/4 " and weigh approx. 3.5 pounds each.
They have few if any bubbles and come 10 billets to a box or they sell per billet I think they have about 11 colors now, They are expensive but great for clear casting.
Pattys or cullet is used for casting also. Mostly clear casting .Irrgreular shapes that need more cleaning (borax)and you have to smash them with a sleage hammer between paper or canvas or something to get smaller pieces.whereas the billet can be cleanly cut.
Check with bullseye. It gos from frit of various sizes (powder to course) to cullet to billet.
I don't kmnow much about shot gl;ass but it is used for casting to. Also plate glass.

About devit from sulfur from plaster. I am confused. I usually cast in plasterWhat to do about it???

I think it is great that you are undertaking this dictonary process.
Please let me know when aI can have one or help.
Judi
ps. if anyone knows how i can spell check on this please tell.

Brad Walker · Post by **Brad Walker** » Thu Feb 26, 2004 10:21 am

lauri wrote:What is the difference between billets and cullets?

-lauri (making dictionary)

Cullet is "(1) Raw glass or pieces of broken glass from a cooled melt; (2) scrap glass intended for recycling. "

As Judi says, a billet is a solid rectangle of glass. The term is used by Bullseye, Uroboros, and System 96.

These terms are covered in the Warm Glass tutorial on this site. The billet/cullet discussion is at: http://www.warmglass.com/Glass_types.htm

Also, there's a fairly substantial glass dictionary online at: http://www.glassonline.com/dictionary/index.HTML (The definition of "cullet" cited above is taken directly from that dictionary.)

Lauri Levanto · Post by **Lauri Levanto** » Thu Feb 26, 2004 4:44 pm

Thanks for help.

Plaster is said to be "aggressive" to glass. I suppose it is
not causing devit but tends to leave a frosty look.
If you add Alumina to the splash layer, you get
clearer surface.

-lauri

Judi Charlson · Post by **Judi Charlson** » Thu Feb 26, 2004 6:19 pm

What is alumina? How much? Where do I get it?
Judi

charlie holden · Post by **charlie holden** » Thu Feb 26, 2004 8:35 pm

Alumina is a refractory that can be used in place of silica, as can olivine sand and zircon. It has to get to a higher temperature than silica before it becomes reactive and starts to bind to the glass. I think, though I'm not sure about this, that silica is also more susceptible to being softened by fluxes that burn out of the glass. Alumina is more expensive. You can get it from ceramics supply houses.

ch

Lauri Levanto · Post by **Lauri Levanto** » Fri Feb 27, 2004 4:01 pm

In addition to what Charlie said
I use Alumina in splash layer because it is
finer grit, giving a smoother surface.
-lauri

Judi Charlson · Post by **Judi Charlson** » Fri Feb 27, 2004 4:42 pm

Lauri,
I usually use talc plus silica and plaster for the splash layer for this same reason. Do you know how they compare?
Judi

Lauri Levanto · Post by **Lauri Levanto** » Sun Feb 29, 2004 11:03 am

Hi Judi,
I have seen complaints that plaster is "aggressive" to glass -whatever that means? If your silica flour is fine I don't think it makes any difference. Talc is a good idea.
-lauri

charlie holden · Post by **charlie holden** » Mon Mar 01, 2004 3:48 pm

I have recently learned that kaolin (china clay?) is about 40% alumina. On "Mixing it With the Best" they recommend adding about 8% by weight of kaolin to other dry ingrediants to help packing, (very small particle size) and improve release from glass. Haven't tried it yet myself though.

ch

Lauri Levanto · Post by **Lauri Levanto** » Wed Mar 03, 2004 3:12 am

You are right Charlie,
kaolin Al2O3 2.SiO2.2H2O conteins alumina.
It has a melting point of 1770 C (=3220F) and therefore
is a good refractory.

The question is about particle size. Alumina in kaolin is bound, and remain so. The Kaolin I can get feels grainy between fingers. Tha Alumina I use feels like potato flour
or maize starch. It is much finer. I guess talc will be a further
improvement. The best packing is achieved with variable grain size.

-lauri

quill · Post by **quill** » Sun Mar 28, 2004 10:37 am

My question: Was the melt too long?@ 1600.
Should it have been shorter and higher(1650).

I am wondering if there is a reason you are going up to 1600?
I know the Bullseye notes mention 1550 & the boxcasting demo I took mentioned this temp as well.

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