Float Glass

Hi just started experimenting with float glass 6mm thick, in the end after kiln carving successfully, had two fire twice more to finally get it to slump then it devitrified!  Any help on a good schedule for slumping into a square mould?  Use a woodrow kiln and the piece was 8" square dropping into a ceramic mould from the states.  First time didn't slump enough, fired again and massively devitrified, my friend says still looks good and someone will love it!  I know you can successfully do this process in just one firing but have yet to find it, any help appreciated.

Hi Lynley,

A GOOD firing pattern is one which gives you what you want from your kiln, having regard to the design power-input and thermal efficiency of the kiln, type of kiln control, and what you want your glass to look like.
Woodrow make many styles of kiln with different levels of control, so just the makers name is no guide. You will need to find a reasonable starting schedule and modify it to suit your need.

Not knowing any of those things one can only offer some starting points from which you can advance with experience. You mention that you have just started experimenting with 6mm glass. Maybe you have already had success with 3mm? If that is the case then simply extending the soak time should compensate for the increased mass and thickness. May take some trial and error to get it right.

As an example of a firing pattern, below is the the one loaded in all Riley GS-2 kilns at shipment as a starting pattern and used in the creation of the platter shown in Basic Kilnforming. Two things must be mentioned;

• That kiln is 25Amps single phase and can heat extremely rapidly, and

• The controller is fully automatic and has wait capability so will wait at the end of each step until the kiln catches up.

 
Step number    Final Temperature                Time for step
                                                                 Hours;minutes
1.  Initial heat           550°C                              00:55
2.  Final heat           780°C                              00:10
3.  Soak                  780°C                              00:30
4.  Top cool             530°C                              00:10
5.  Anneal soak       530°C                              00:20
6.  Anneal cool        480°C                              00:20
7.  Final cool             80°C                              02:00

Not all controllers have wait capability. See the sections in All about kilns.
If your controller does not, then you must enter a time which is longer than the capability of the kiln to change temperature.

The GS-2 is a Top Hat style, with low heat loss. The thermocouple is fairly close to the glass. In front loading pottery style kilns the thermocouple may be high and the glass low down; so the difference can be great.  A higher indicated soak temp will usually be needed with front loading kilns.

There is a balance between edge development and conformity to mould or design. The longer the soak time the better the mould conformity. However, the longer the soak time the more the opportunity for the edge to change shape. Remember the effect of equilibrium thickness to influence the flow of the glass. Strive for a balance. Edge shape is more readily discernable than design conformity.

Devit will occur at that temperature but will rarely be evident in soaks of less than 2 hours or so. Be sure and clean the glass thoroughly before firing as dirt and smears do encourage its development. 

Are you sure it was devit. Could it have been tin bloom? Most likely was devit as you say it appeared only after previous firings.

Float is cheap. Rather than multiple firings of a single piece, start with a new piece each time and vary soak time or temperature until you understand what happens in your kiln. Alternatively; set in a long soak time, monitor the drop and advance to the next step when happy with the detail. Hard to do with hot glass but still frequently done.

Good luck and happy times.

Peter Riley

Hi Peter,

Marlene here, the proud new owner to be of a Peter Riley kiln. I to have been experimenting with 3mm float and have encountered devit. My first two peices were fine, but every other firing since has seen devit form. I am currently using a tetlow front loader 10amps 33cm x 33cm x 45cm with a manual temp controller. Can you please supply me with a firing schedule for my tetlow kiln and also for when I get my new kiln. I will be using 3mm float. Look forward to hearing from you.!

Previously marlene kasper wrote:

Hi Peter,

Marlene here, the proud new owner to be of a Peter Riley kiln. I to have been experimenting with 3mm float and have encountered devit. My first two peices were fine, but every other firing since has seen devit form. I am currently using a tetlow front loader 10amps 33cm x 33cm x 45cm with a manual temp controller. Can you please supply me with a firing schedule for my tetlow kiln and also for when I get my new kiln. I will be using 3mm float. Look forward to hearing from you.!

Hi Marlene,

First, to put the record straight for all, I'm getting too ancient to build kilns any more so your new kiln is a Custom single hearth kiln designed by me to your specification and built by Ge and Ge kilns of Monbulk Victoria. Would hate people to think I was telling porkies when they enquired about GS or FS kilns and were told that I had retired. 

Second. Devit. Am surprised that you are experiencing it so regularly on 3mm float. It is easily confused with tin bloom. It's best to put the tin side down as there it is cooler than on the top side and can get lost in the texture.

There are at least three ways of finding the tin side. The few, such as my friend Helen W, can detect it just by looking. Others use a fine water spray and observe the way the water trickles down the inclined sheet of glass. Most of us use a UV lamp to fluoresce the tin. The fluorescent torch used by many to make their own is no longer available and I am awaiting the importer to give me a sample of the replacement device. Then, details of the conversion will be published alongside those for the polariscope.

Third, There is a starting pattern for thin float glass loaded at pattern 1 in the Shinko controller on your new kiln. You can modify it as you wish when you see how your initial firings come out.

Fourth. A pattern for a Tetlow kiln. It depends on the degree of complexity of the kiln controls. Pottery kilns often have only the most rudimentary controls. If they do have a means of controlling the rate of rise they may not have wait capability. That would mean that it could turn off the power when the controller reaches soak temp; not when the kiln gets there. Potters don't need a soak time so there may be no way to set that in. 

Here's a rough guide;

• Load glass on one shelf at about the same level as the thermocouple.

• Often they don't heat up very fast and the fastest rate of rise could be tolerated by float, so set the soak temp to around 800 degrees C.
  
• If a soak time can be set then enter about 30 minutes. If no soak time can be entered then increase the soak temp to about 840 degrees C

The reason for the latter increase is that the kiln will take some time to rise from 800 to 840 and that's about the same as a soak period. 

Cheers, Peter R

 

A further point on resolving confusion between tin bloom and devit is that tin bloom makes the glass cloudy without affecting the surface smoothness whereas a devitrified surface will become progressively more rough as it progresses. I am told that a sensitive touch can detect the roughness of devit at an early stage but I have to take that on trust.