Fibre Paper Re-use in Kiln Carving

“I would like to use 1/4 inch Fiberfrax to impart texture on the back of transparent glass.  Is there a way to make it reusable?  I tried mould hardener on a small piece of it, but the hardener wasn’t absorbed.  I’m afraid the fiberfrax will lose its structure if I pre-fire it (to burn out the binders) before removing it to apply hardener.”

 

It is difficult to reuse refractory fibre paper after moving it between firings, but not impossible.  I have used two processes. One is to place the glass over the cut fibre paper. This works for small pieces. The fibre paper was placed on thinfire to allow air migration out. I used a long low temperature bubble squeeze to ensure the binder was completely burned out.

The other arrangement I have used for larger pieces.  This is to assemble and fire the fibre paper to burn out binders.  There is a large chance that not enough air will get to the centre of the fibre paper when large glass is placed on top.  Binder not burned out leaves a brown mark on the fibre paper and stains the glass grey. Turn off the kiln once all the binder is burned out as evidenced by the paper returning to white.  As soon as the temperature in the kiln is comfortable, you can place glass on top of the fibre paper.  It is strong enough that it will not be compacted by the weight of the glass.

Using un-hardened fibre

But there is no logical reason for these processes, although they work. Firing to about 500ºC/930ºF with a suitably long soak will clear those gasses before the forming of the glass begins.  You will know when there no longer is a smell of burning paper, or on more recent fibres a chemical smell.  Make sure you vent the kiln during this burnout to allow the smoke to escape.  For a large area, a soak there may need to be hours long.  Another check is when the fibre has turned white again, the binders are gone.  A further protection against bubbles in any area is to place the whole assembly on a bed of fibre paper.

It is possible to use un-hardened refractory fibre without a separator, as the older versions do not stick to glass easily.  However, if you are using the current eco fibres, they will stick in many areas.  Kiln washing any fibre before firing is the best protection against lengthy clean ups.  It also allows the best chance to remove the un-rigidised fibre for re-use.

A smoother surface can be given to the refractory fibre, if you want. Do this by smoothing powdered kiln wash over the bed layer and any other layers the glass will be touching.  An alternative to powdered kiln wash is to put Thinfire or Papyros cut to shape over each layer.

After firing, slide the fibre onto cardboard or another flat stiff surface. Then place into a large pizza or similar box. I have stored fibre in such a way for several firings.

Using hardened fibre

Of course, the fibre can be hardened by use of colloidal silica.  Make up the whole stack of fibre paper for the kiln carving.  Harden the whole stack at once. This helps to bind the layers together.  Brush on the hardener to the exposed part of each layer.  Cover both horizontal and vertical surfaces. Hardener does take time to soak into the fibre paper.  Give it time.  You can add more hardener at intervals.  Be careful to avoid overdoing it.  Fully wet fibre is difficult to move and takes a long time to dry - days.  The objective is to harden the surface of the fibre, not to harden the whole by soaking it.

Allow the carrier of the hardener to evaporate for hours or a day.  When you can move the fibre, fire to at least mid-700’s ºC/ 1300ºF to 1400ºF.  After firing, it must be covered with significant amounts of kiln wash. This can be as a liquid or as powder. I prefer liquid.  The kiln wash is required over all edges and surfaces to keep the glass from sticking to the fibre.

Storage of the rigidised fibre paper can be in the same way as for the fibre without binders or hardener.

Texture Moulds

 Texture moulds are a form of bas relief in reverse. The texture of the mould is the bas relief. The glass formed over the shapes is in negative relief. The light is refracted through th
e back to give an image of bas relief although the surface is smooth.

 

Example of wave form texture mould

 These moulds are prone to produce bubbles at the generally recommended tack fuse temperatures. The glass often sticks to the mould if there is not sufficient draft to the parts of the image, or if insufficient separator is used. Often the moulds are produced with a rim around the edges, which trap the glass.

 The usual temperatures are too high. These moulds are an exercise in patience. The temptation is to fire higher than slumping temperatures to get good definition in the glass. However, a number of problems, especially bubbles, can be avoided by staying at the high end of slumping temperatures. This means the top temperature would be about 680C. To compensate for this low temperature, the soak needs to be three hours or more. To be sure the definition desired has been achieved, peeking near the end of this long soak is necessary. 

 Moulds that are produced with a rim around the edges can trap air and create bubbles. The rim forms a perimeter dam to confine the glass. If the rate of rise is quick to a high temperature, the edges can be sealed against the rim before all the air has escaped. It is advisable to cut the glass for these rimmed moulds a bit smaller than the internal dimensions formed by the rim.

 

Example of textured area surrounded by a rim

 Use of single layers on texture moulds can lead to large, thin bubbles. This is most prevalent when using high temperatures. Since the single layers tend to form more slowly than an already fused two-layer piece, the temptation is to use higher temperatures. The higher temperatures soften the glass to such an extent that often bubbles form over the lower areas of the mould. Instead, low temperatures with extremely long soaks should be used to allow the glass to conform to the undulations of the texture without dog boning or developing bubbles. Of course, peeking will be required to determine when the texture is achieved. With single layers, the surface will have greater undulations than with two layers. The thinness of the single layer cannot fill the depression the way two layers can.

 

 Rapid rates to high temperatures can produce internal bubbles too. These moulds have a multiplicity of hollows and depressions. Just as people are warned about depressions in their shelves, the depressions in the texture moulds can cause bubbles too. This means there are even more possibilities for bubble creation than on apparently flat shelves. Long slow bubble squeezes are required to allow air from under the glass.

 Glass sometimes sticks to the mould. This is most often blamed on insufficient separator. Boron nitride is a good separator for these moulds especially if you go to tack fusing temperatures. At slumping temperatures, kiln wash will normally be sufficient. Both of these separators need to be applied carefully, as there are relatively steep slopes throughout the mould. Spraying needs to be done from at least four angles to ensure all the sides are covered.

 Painting on kiln wash is a little more difficult, as the solution is so liquid, it tends to run down the slopes without much sticking. One means of rectifying this is to tip the mould in a circular motion to move the still liquid kiln wash solution around the slopes.

 Less often thought about is the draft of the shapes of the mould. If the slopes (draft) in the mould are too steep, the glass will “grab” the ceramic mould, because the ceramic contracts less than the glass when cooling. If shapes of the mould are steep and deep enough, the glass may even break as a result of this compression of the mould.

 

An example of some nearly vertical elements and a rim

 Of course, if a flat front surface is required, a higher than slumping temperature must be used. This is required to allow the glass to flow to the lower portions of the mould. It still should be as low as possible, but with very long soaks.

 Avoidance of bubbles on, and sticking to, texture moulds is best achieved by avoiding high temperatures, use long soaks, use two layers, and avoid extending glass to the rim. These combined with observation of the progress of the firing will produce the best results.

 

Other information is available:

Low Temperature Kilnforming, an Evidence-Based Approach to Scheduling, an ebook

Bas relief

Layups promoting bubbles