Help with kilncasting small glass pendants

[Percy pigeon]

Hi Everyone,
I am new to this forum.
Lived for five months in Alderney and work most of the time in porcelain , making my own plaster moulds and models.
Love making small designs and have made my own glass clay, decided it is not transparent enough  for a quality glass product.

Been making rubber moulds of my wax models out of silicon sealant for bathrooms.
Casting wax models from these from a green wax for lost wax casting.
Then making a plaster mould of these and steaming out the wax after the mould has cured for a few days.

Up this point everything is fine because making models ,moulds, firing kilns with programmers and putting in different firing scedules is what I have done for years , but once it goes into that kiln my problems begin !

Can anyone help me , my attempts at kiln casting is now turning into a nightmare.
This roughly what I have done so far

I am trying to cast some small glass pendants about an inch long by inch wide.
With the first firings I used medium bullseye frit, in pre-dryed moulds, which I make from warm-glass's plaster. Fired over 830 C trying  to get the frit to be flat on top.
The casting although clear had not melted in places against the mould wall because off the chunks of glass being too big.

Then I read that to get the cast flat on top and to get a smoother casting to use fine frit. I did not have any, so decided to make my own from the medium frit I had , the enameling pestle and mortar and sieves I have.
I made some  finer frit, colour it and fired it, but these were too cloudy and still not flat on top. Glass Seemed to have pulled away from the edges at the top taking plaster with it.

Read somewhere that if the moulds are not dry enough can cause problem with castings.
So thoughly dry moulds , I sieved out all the glass that would go threw my 80 mesh sieve, which was guiet a lot. Also  what would go threw a sieve which has no size on it,[ but I would say that the holes are about 0.75mm.]

With what was left then put 10% of coloured bullseye glass powder in.

I then fired this to  794 C soaked for twenty mins. Saw that the top had melted and pulled away from top.
Finished firing . Refired the next day after adding more frit to the top to make a flat top.
Fired to 741 C and melted the frit. Annealed and finished firing.

Next day broke mould and took the peices out of the mould. They were flat on top, but the glass against the inside of the mould had not melted at all. My partner pointed out that the hot was getting too hot and the bottom was not hot enough ?

I remembered that in the last few firings I had not lifted the mould up as high off the kiln floor as the first firings .

So filled another mould lifted to centre of kiln. This  time fired very slowly to keep temperature even .

At 740 C top was still flat and not melted, so took kiln up very slowly at about 1c a min, stopping at different temperatures to give ten or twenty min soak. At 890 C frit still had not melted. I gave it a soak for about 30 mins but still did not look like frit had melted.
Anyway since I seemed to be able to see threw the casting  to bottom of casting now, thought perhaps some of it had melted. So switched kiln to go into the annealing cycle and then go off.

When I opened the kiln this morning , the top is more or less flat, the glass is fused together and transparent as it can be, considering of it has not melted and flowed. They are like they transparent clear sugar cubes. Very good as sculpture, but not for pendants.

Now is this because I fired too slowly ? or is it because I sieved out all the smaller particles so the particles were all the same size, bearing in mind though I did add back 10% powder in the form of coloured glass powder.

Anyone help or know the answer.

I guess I go back to my first firing schedule which took the glass up fast as possible between 677 C to  816 C , have the mould in the centre of  kiln and see what happens.

Unless anyone has any suggestions.

I am just trying to make a small clear pendant with a flat back and picking up all the detail in my model ! Maybe it is not possible

[GlassOwl]

One of the ways I have done small pendants with Frit is using Frit, Freeze and Fuse.

I make a paste from the very fine powder frit and a water /suffactant mixture, pack it really really tightly into the mould tamping it down each time, then in the freezer until frozen and then take out of the mould carefully whilst frozen, put on kiln paper and fire whilst still frozen. Of course if you let it de frost it will collapse.

I have managed a few interesting results using this method but my clear glass is never that clear.

Havent got an image to show you but these pendants were hearts about 1.5 inches by 2 inches by 6mm deep

Hope that is ok

Sandra

[Black Heart Beads]

It has been many many years since I did cast glass and don't have a good enough memory to advise on the kiln schedule.  I did quite large scale kiln cast pieces but expect the same applies on a smaller scale.

I would try using a larger piece of glass rather than frit if you want it clear. Frit will trap tiny bubbles between the grains and you also often see faint ghost images from the grains when they have melted together. Using as few large chunks as possible should give a clearer look but may not give a totally flat back, tweeking the schedule may help this but I would expect to do some cold working. You didn't mention the design you have on the pendant, if its very detailed that may also cause a problem.

If what you are making needs frit and you are making your own don't forget to wash it (you may have done this).

Do let us know how you get on.

[sarah t]

hello and welcome

a course with either

Rachel Elliott  @ glass works     http://www.rachel-elliott.com/
or  
Dawn Turner      http://www.dawnturnerdesigns.co.uk/

might help

[Moreton]

First thought is what Samantha (Black Heart Beads) said. The frit is too fine for a clear result. The bigger the pieces you put in the mold the clearer it should be. Have you tried using a container, flower pot for instance, to drip feed the molten glass into your molds. That gets over the uneven melting as glass and mold are at the same temp so no cooling effect at the edges. If you are loading the mold with glass "chunks" try keeping the glass away from the mold's edges so that molten glass flows to the centre  rather than up the sides. Most kiln formed glass does need after work such as grinding & polishing. Sometimes that takes longer than the rest of the process.

Hope that helps show us how you get on. We won't laugh................Honest
Pete

[Percy pigeon]

 Thanks for all your advice. I wonder if I have got a bit of devitrification in there, since I was over 700 C for a long time and/or should have soaked for longer at top temperature .Taking it up too  slowly would, mean I was a long time over the 700 C.
A lot of firing schedules go AFAP from say 677 c to the process temperature.
Also the possibility that the mould mixture had too much plaster to water ratio.  I do weigh out the plaster , but my digital scales are tempremental and I noticed that some of the plaster as I mixed it seemed much thicker than for other moulds (I made from the same plaster).
I am not too worried about the transparency I have achieved. By sieving out the finer glass particles from the fine frit ( those that do not go threw 0.75 mm hole sieve) I have made the castings nearly as transparent as using the medium frit. Also the the colours seemed to be more evenly mixed than when using the medium frit.
    By the way and changing the subject slightly after sieving the fine frit threw the sieve with the 0.75 holes I then sieved it again threw a 80 mesh sieve.  I made some glass clay out of this hoping this would be more transparent than using powder. It made a very course clay, which I would not like to model with, but was able to press into one of my rubber moulds and freeze and take out hour later. Although couser that the clay made from the powder , it had picked up all the detail.
Later fired this to 725 with soak of 10 , which produced a shiny very translucent , but not transparent peice. You could see a lot of bubbles in it.  I have seen on another forum that putting a bubble squeeze in a firing even with powder frit can get rid of all the bubbles if long enough soak.
So my next firing for the glass clay made with frit between 0.75 mm hole and 80 mesh will include a bubble squeeze and may take it up higher , and have no soak.
Let you know how I get on.

[Pat from Canvey]

In order to get glass to flow, I fire pot melts at 930 deg centigrade to get a smooth flat disc of glass that can be slumped later into a mould. Your temperatures seem too low to me but plaster molds probably won't stand up to the 930 temp. I thought I read somewhere that you have to add flint to plaster molds that can be used only once. If I want a mold, I use white clay and fire it to bisque first in my ceramic kiln. If you want to see a bit of blurb about pot melts see
http://kiloalphatango.com/latest_pot_melt
A simpler way of getting pendants made is to cut sheet glass to shape, stack the shapes, add any frit or stringer decoration and fuse flat. Have you seen http://www.warmglass.com/basic.htm
There are some great tips there.