Placing of Pieces in the Kiln

 Distance from Sides of Kiln

 

"Is there a rule of thumb for interior size of kilns and piece size? (i.e., “allow for X inches between the piece and kiln walls on all sides”).  I’m thinking about how to determine piece size limitations when shopping for a kiln."

I don’t know of a formula, or rule of thumb, to determine the amount of space required between the glass and the kiln walls.

I have only been able to determine the spacing required after I have purchased the kiln.  Each kiln has different characteristics. 

The most obvious is whether the kiln is fired from the side or from the top.  More space is required with side fired kilns.  The radiant heat from the elements tends to heat the edges of the glass before the centre becomes equally hot. This requires more space or baffles between the elements and the glass.

Top fired,  with enough distance to get even distribution of heat

Side fired. Red arrows indicate the important infrared heating.
Blue arrows indicate the less effective ambient heat.

There is less concern about uneven heating with top fired kilns.  But as each kiln is different, you must test the heat distribution around the kiln.  Bullseye Tech Note #1 has a good method.  This will show where the temperature is less than the rest of the shelf.

In general, rectangular kilns are cooler in the corners.  Round kilns do not have the same characteristic, but may still have uneven temperatures, due to the configuration of the elements.  Smaller kilns seem to have more even temperatures than large kilns, which tend to be cooler along the sides.  Kilns with a ring element below the shelf seem to have the most even distribution of temperature.

I had a large kiln 2 metres by 1 metre which had a requirement of 50mm/2” from the edge to even the temperature.  A recently purchased 50cm square kiln has almost perfectly even temperatures across the whole shelf.

[The illustration is taken from the ebook Low Temperature Kilnforming, available from Bullseye and Etsy.]

The required glass distance from the side will depend on side or top elements and size but no formula is available.  Testing for heat distribution is necessary once you have the kiln.

Changing size in Slumping

 “I have full fused a single piece of glass with a few small pieces on top.  I thought it would shrink some as I had been told, but it maintained its size and still fit the mold for slumping.” 

I believe the enquirer is talking about a single layer circle changing size at full fuse.  Dog boning is much less evident in circles than rectangles.  The glass retreats evenly all along the edge.  This gives the appearance of retreating less than rectangles.  The absence of any big change in size may also result from thinning of the centre.  The amount of size change will be affected by the temperature of the full fuse too.  In this case there were additions which will have resisted any tendency to shrink.

Lower top temperatures, more rapid ramp rates to the top, and shorter holds will have the effect of limiting the movement of glass toward 6mm thick.

credit: Bullseye Glass Co

The viscosity of glass at full fuse is enough for it to attempt to pull up to 6mm. At casting temperatures, the viscosity is so low that 6mm of glass spreads out.  Temperature affects viscosity.

 

At slumping temperatures (620˚C - 680˚C / ca.1150˚F - 1260˚F), the viscosity high enough that the dimensions of a circle do not change. A circular piece of 3mm glass held at slumping temperatures does not change dimension.  It may, if held long enough take on a kind of satin sheen, rather than a fire polish.  But the viscosity  is low enough to allow the glass to form to the mould, given sufficient time. The resulting slumped piece will appear to be smaller than the mould. If you measure the piece around its outside curve, you will find the distance is almost the same as the diameter of the blank. 

 

Changing size on a single layer piece is dependent on the temperature and heat work applied to the piece.

Bowl Split Analysis

The visual evidence relating to this enquiry is a sharp-edged break through the middle of the slumped piece. The two parts have slumped separately, and seem attached at the rim, leaving the middle opened.  A moderate slumping temperature was used to fire the piece at the bottom of a stacked kiln load.

This is used as an example of the kind of thinking required when investigating breaks in slumping.

The split occurred before the slump was complete. We know this because the pieces no longer fit exactly together.  This means the crack opened as the slump continued.  There is other evidence.

The opening of the crack cannot have happened at or after the annealing. It would have already formed to the mould in a whole state. It would break completely across, because it would be in a brittle state.  And the pieces would fit exactly together.  But they do not.

This piece was at the bottom of a stack of shelves in a deep kiln.  At the bottom, there is no radiant heat, only side heat.  This could be a major cause the kind of break described.

It is possible that the split was not across the whole piece.  At the bottom of the kiln, the glass is not receiving any radiant heat from the top.  It is getting radiant heat only from the sides of the kiln.  That means the edges were considerably hotter than the centre.  The edge may be in a plastic state while the centre is still in the brittle state.  The contrasts in expansions are often great enough to break a piece.

From the evidence we have, it can only be said the ramp rate was too fast for the conditions. 

This little exercise shows that a lot of information about layup, schedule, place in the kiln, and any other relevant variation on the usual, must be detailed when asking why something has not turned out as expected. 

 

 

Uneven Slumps on Deep Rectangular Moulds

 "Can anyone please tell me why this mould always comes out wonky and devitrifies and pulls in on the edges. I used Primo Primer; my kiln is level, and this is the slump schedule I use for 3mm base with 6mm in places [temperatures in Celsius]: 100/593/30 mins; 66/663/25 mins; 204/482/60 mins; 66/371/10 mins; END.*   12cm square."

 GlassAlisa

The suggestion has been made that having a 6mm base would lessen the irregular slump in the mould.  I am not convinced that making the base thicker will sort the problems.

 

1)  This is a very deep mould in relation to the span.  The mould sides are steep.

 

2) The glass slides down and picks up marks from sliding down the walls of the mould.  The marks are not devitrification.

 

3) Deep slumps are prone to going off centre. One fix is to watch and be prepared to reach in with wet sticks to readjust the glass placement on the mould.  

 

4) Deep moulds (deep is relative to the span of the mould) require two or more stages of slumping. Start with shallow a slump, and progress through steeper ones.

5) The sides dog bone on many rectangular moulds.  One way to reduce this is to round the corners with a 10mm radius.

 

6) Reducing the forming temperature, and extending the soak time dramatically, will go some way to alleviating the previous problems. I suggest trying a 620C slump temperature and soak for 2 - 3 hours.  Peek at intervals to see when the slump is complete, then advance to anneal and cool.

In my view, it is a mould from a maker that does not fully understand glass behaviour.

And in passing, the ramp speed from top temperature to annealing should be as fast as possible, to avoid any risk of devitrification on the way down.  

* Schedule in Fahrenheit for the Americans.  

• 198 to 1100, 30' 
• 119 to 1225, 25'
• 367 to 900, 60'  [ASAP is the recommended rate.  As it is a tack fused piece, I would anneal as though 12mm/4 layers.  This would use a 2 hour soak, cool at 100 to 800, 180 to 700, off ]
• 120 to 700, 10'
• End

Multiple Layers of Kiln Wash and Fibre

Recently, there have been confirmations of multiple of layers of kiln wash on the shelf under pot melts, frit stretches, and flows.  Ten, and even sixteen layers are mentioned. Also reported are two layers of 1mm fibre paper as a separator for the same processes.

These practices are excessive and wasteful.

 

Kiln wash   

·         Once fully covered, the shelf does not need additional layers. 

·         Stir the kiln wash mix each time you dip the brush.

·         Apply thinly.

·         Use only enough coats to evenly cover the shelf.

 

Fibre and shelf papers are not recommended to be placed on the shelf for high temperature processes.

·         The shelf papers can become incorporated within the glass as it moves along the shelf.

·         Fibre papers inhibit the movement of the glass in unpredictable ways.

 

If you do use fibre papers anyway:

·         Use only one layer.

·         Place a clear sheet of glass over the fibre paper to allow better flow during the firing.

·         A disc of clear glass also helps to separate opalescent glass from the shelf.

 

More layers of kiln wash or fibre paper does not make a better separator.