theanimallover wrote:Hi,
Further to my previous post. I am having problems with devit on sky blue and spring green transparant glass, it shows up more on the the green I believe as I back with white which illuminates the glass (and the problem).
When I fire on my bubble squeeze program the glass has no bubbles but devit, if I use my preprogrammed 'slow' it has no devit but bubbles! Please can someone help with a new in-between program to try?
The bubble squeeze:
300 750 10
300 1150 30
50 1250 2h
300 1480 15
Slow fuse (no devit but bubbles occur) I use the skutt programmed program. I can't find info on what it exactly is, but I think it is this one:
300 250 25
300 500 25
300 750 25
600 1250 20
I really appreciate your help (in advance
)
Thanks,
Karen
I have looked at your schedule again, and noted how long a soak you had at 677ºC. After a slow advance from 620ºC a 2 hour soak is excessive. It is at the bottom of the devitrification range so can prepare the conditions for devitrification. A maximum of 30 mins is all that is required at the bubble squeeze.
In your second schedule I find the soaks unnecessary. Glass performs best with regular steady advances in temperature. So, you can alter the schedule to give that steady rise by thinking about how long the kiln takes to achieve the temperature you require. You are essentially climbing in temp at 300ºF up to 750ºF, then advancing rapidly to the top temperature, but without a bubble squeeze. Also 750F is below the strain point of the glass, so you are taking risks with thermal shock to the glass.
In the schedule above you are taking 3.75 hours to get to 750F. you can get there in the same amount of time by using 200F/hr and this will have a smaller chance of shock. It is of course, relatively slow - you could take it up at 200C (360F) per hour safely.
However, you have no bubble squeeze of any note in this schedule - which occurs at the softening point of the glass - between 620 and 677ºC (1150 and 1250F). 333ºC/hr is simply too fast to allow the glass to reach the softening point satisfactorily. The slow rate of advance in your first schedule would work with only a 20 min soak, or simply a stop for 30 mins after a 200ºC rate of advance to 677ºC.
If you are using Bullseye kiln wash and firing below 1500F for 10mins at top temperature, you should not have any big problems. But remember you need to renew your kiln wash each time you go above about 730ºC. A chemical change occurs in the kaolin - a constituent of the kiln wash - at around that temperature. There are other kiln washes, of course. Primo works well in my experience, so will a solution of alumina hydrate, CMC and water.