Glass not Melting

[art4ul]

I just casted a head shape piece that is approx. 12" tall 9" wide and 3.25" thick. The piece was casted inverted with the neck area to the top. I filled the cavity with Bullseye glass(chunks and frit). Fired to 1500 degrees. Glass in neck area melted but when I opened the mold the rest of the glass was UNTOUCHED! The other pieces in the kiln all seemed to do fine(open molds). This piece seemed to have been in a different universe during the firing. Any ideas on what happened?

[charlie]

not hot enough for long enough. it takes a LONG while for the heat to get through a thick mold.

[art4ul]

Hi Charlie,

How long is long enough?

The mold by the way was 2.5" thick done in three horizontal sections and reinforce with chicken wire with additional mold material. The firing was held at 1500 degrees for an hour. I had done a visual check to see if the glass had filled the mold.

[charlie]

Hi Charlie,

How long is long enough?

The mold by the way was 2.5" thick done in three horizontal sections and reinforce with chicken wire with additional mold material. The firing was held at 1500 degrees for an hour. I had done a visual check to see if the glass had filled the mold.

i'm not a caster, so will leave it to others to comment on rules of thumb. it's just that everything i've read about this matter says that 1500 isn't hot enough, in that you want the glass to be really liquidy. for example for pot melts, the top temp required is 1600-1700 to get the glass to go through the hole at the bottom of a clay pot. 2.5" of mold is really a long distance to get the heat through.

you also have to have a way for the air to get out of the bottom of the mold. the top of the cullet will melt and form a plug preventing air escaping.

[rachael r]

I looked up the firing scheduale for a up to 6" thick casting. Here is an 18 hour schedual and a 36 hour (for a thick mould, don't know if 2.5 " is "thick") here goes:
60 dpm to 320 4.5 hours
hold 1 hour
106 dpm to 1330 9.5 hours
hold 2 hours
up to 1560 in one hour
hold for 6 hours


the other is
ramp to 320 over 6 hours
hold for 4
ramp to 1330 over 10 hours
hold for 6
ramp to 1560 in 1 hour
hold for 10



i recomend the longer one as it may allow more air to escape and i error on the side of caution but if your mold isn't that strong...because you already fired it once...you may want to remake the mold or use the shorter scheduale. the mold soaks a lot of heat that is why your open face molds were done but the closed one wasnt
here is an annealing scheduale too:
soak at 960 for 8 hours
cool to 580 over 14 hours
cool to 490 over 10 hours
cool to room temp over 16 hours
these cooling times might be flexible... but the soak time probably isn't

i just took all these out of my notes but they were given to me by a repuatable caster so i trust them,... sorry if I am full of crap and dont know what i am talking about... if so someone will tell you hopefully.
also are you using a flower pot on top to make sure your mold is full of melted glass? frit an chunks hold lots of air
good luck

[rskrishnan]

I just casted a head shape piece that is approx. 12" tall 9" wide and 3.25" thick. The piece was casted inverted with the neck area to the top. I filled the cavity with Bullseye glass(chunks and frit). Fired to 1500 degrees. Glass in neck area melted but when I opened the mold the rest of the glass was UNTOUCHED! The other pieces in the kiln all seemed to do fine(open molds). This piece seemed to have been in a different universe during the firing. Any ideas on what happened?

Welcome to my world of "learning from failures" !! Yes, I've been there and felt the "Oh My *&%$ing God" feeling when you open the mold (which seems to have held up pretty nicely!) and then find nice dry frit sitting where you expect solid glass !! There is no bigger anticlimax possible!!
But .... it's one lesson I <and perhaps you> will NEVER forget. I made a series of "thin" castings - i.e. smaller moulds and then jumped onto a bigger one similar to your piece's size. The only problem being I stuck to my "small piece schedule" - result ==> DISMAY!

To add to the misery this was the 4th time I was making this piece - previous attempts failed for reasons ranging from mould breakage, not enough glass to fill the mould, not a well cured mould .. etc etc.

I thought I was alone in facing such problems - but it is a very useful lesson in "think before you fire" ( and one not easily forgotten).

Expect to try my piece again next weekend!

Krishnan