A Sintering Project

Ready for firing

The project is to fire 6mm “balls” stacked 3 high onto a single sheet of clear glass without significant alteration to the base sheet or to the stacked balls. This creates a total thickness of 21mm. The proposal is to sinter the whole in one firing.

Scheduling for a sinter firing needs to be done as though 2.5 times the thickest part – in this case 52mm, or 2 inches

It is slightly more risky to do this in two firings, than one, in my opinion. A suggested schedule for sintering frit using Bullseye was:

• 100ºC /180ºF — 482ºC /900ºF, 60'  =5.8 hrs

• 40ºC /72ºF — 593ºC/1100ºF,10'      =2.8

• 20ºC /36ºF — 665ºC /1230ºF,30     =4.1

• Skip to anneal temperature, soak for 6 hours =6.5

• 6.7ºC /12ºF — 427ºC /800ºF,0'       =8.2

• 12ºC /22ºF — 371ºC /700ºF,0'        =4.7

• 40ºC /72ºF — room temperature,0’ =8.8

• Off                       =40.9 hours total or 1.7 days

This was annealing as for 38mm/1.5 inches thick. Annealing for 50mm/2” thick would need about 112 hours or 4.6 days.

However this schedule was not successful – the pieces were only lightly stuck together. Thinking about why, led to the proposal that the soak time and temperature were not long or high enough to give adhesion between the pieces.

A second attempt used a faster ramp rates to higher temperatures.

• 200°C /360°F – 540°C /1004ºF, 30’ =3.2 hrs

• 60°C /108°F -625°C /1157ºF, 30’     =1.92

• 30ºC /54ºF - 685ºC /1265ºF, 120’    =4.0

• skip to anneal temperature and soak/hold for 4 hours (as for 25mm/1”)

• 15ºC /27ºF – 427ºC /800ºF, 0’        =3.67

• 27ºC /49ºF – 370ºC /700ºF, 0’        =2.11

• 90ºC /162ºF – 50ºC /122ºF, 0’       =3.56

• Off

•  = a minimum total of 18.5 hours plus natural cooling of the kiln

This schedule used a:

• faster first ramp to a higher (540ºC /1004ºF) first soak

• a faster (60ºC /108ºF, which is 150% of the previous) rate to the lower slump temperature (625ºC /1157ºF)

• the same relative reduction (50%) in rate to a higher temperature (685ºC /1265ºF)

• a shorter (120’) anneal soak

• and consequently faster cooling rates, which showed no stress after firing

The whole structure held together and was sound. There was no apparent change in the size of the individual 6mm balls.

This difference in scheduling is an illustration of how time and temperature can be interchanged.

It also shows that size matters when sintering pieces together. Higher temperatures and more time are required for dots and balls than for frit.

More information is available in my e-book Low Temperature Kilnforming, available from Bullseye, Etsy and stephen.richard43@gmail.com