Preventing dog boning

Firing a single layer, even with decorative elements on top, is most likely to “dog bone” due to lack of volume.  With a single layer you are always going to have difficulties with volume control. 

Photo credit: Paul Tarlow

Unless you are satisfied with an angular tack fuse at the lower end of the tack fusing range, you will always run the risk of dog boning. All the other variations of tack fusing use increased temperatures causing the glass to begin to pull in along the long sides to a greater or lesser extent (more with contour fuse, less with angular tack fuse).

Dog boning occurs because as the glass softens and the edges begin to round, the viscosity takes over from the solid phase of glass as a major force.  Viscosity can be thought of as an approximation of surface tension. 

Glass is a material with a plastic range over several hundred degrees.  This means that the hotter the glass becomes, the less stiff it becomes, and the viscosity force thickens the glass toward 6-7mm in the kilnforming temperature range. The greater the temperature, the more the glass pulls into a ball shape, or in the case of sheets, thickens at the edges and thins in the middle.  Higher temperatures reduce the viscosity to the extent that it becomes as thin as one millimetre.

Trick the glass

To avoid dog boning on tack and full fusing, you have to trick the glass with some special scheduling.

The trick employs the concept of heat work.  The nature of glass allows you to put a lot of heat work into a piece by soaking for a long time at a low temperature.  You might think of it as a kind of sintering.

A description of sintering:

The atoms in the [glass] diffuse across the boundaries of the particles, fusing the particles together and creating one solid piece. [This can be done by heat at low temperatures with extended soaks.]  Examples of pressure-driven sintering are the compacting of snowfall to a glacier, or the forming of a hard snowball by pressing loose snow together.   https://en.wikipedia.org/wiki/Sintering

By sintering (sometimes called fuse to stick) or - in kilnforming terms - by the use of heat work you can achieve the result you want without dog boning.

By taking the temperature slowly to about 700°C to 720°C and soaking there for two to four hours you can achieve a rounded tack fuse without dog boning.  You will have to experiment with the exact temperature and length of soak to get exactly what you want.

The length of soak time or exact temperature is not vital.  The two in combination will achieve the effect you want.  The importance of observation of your firing is re-enforced in the cases of sintering.  You cannot be sure until you check during the firing whether the edges of the glass are rounded enough for your purpose. That observation will also tell you whether a slightly raised temperature would be useful.  You will learn the time required to achieve the effect by recording the soak time when you advance to the anneal soak and cool.

Further information is available in the ebook Low Temperature Kiln Forming.

By the use of heat work in kilnforming you can achieve tack fused pieces without dog boning.