Spider Web Cracks

 

Credit ASTM

 The nature of the cracks - and spider web describes it perfectly - shows an adhesion problem. It is not an annealing problem as that shows a single sinuous line with a hook at each end. It is not a compatibility problem, as that shows as cracks or breaks along the edges of the combined glasses. It is not a thermal break, as those show as breaks where the glass has separated to some amount.

 

Glaze crazed in a ceramic vessel

 

The cracks are exactly like crazed glazes on ceramic objects. And for the same reason. The glass is trying to contract more than the underlying ceramic. It is stuck to the pores of the ceramic and creates a crack where there is a slightly weaker part of the glass. These cracks in ceramic glaze propagate across the surface as it wears, or in the kilnforming case as it cools.

 

Glass puddled in ceramic

 Most usually it results from a lack of separator in that area of the shelf, or uncoated kiln furniture. It indicates either the glass has adhered to the shelf or mould, or (rarely with fusing glass) that the glass has suffered severe devitrification.

 

 

 Occasionally there will be the appearance of shards of glass. This will be where the glass has stuck to some particle on the shelf. Sometimes it can be a speck of something resistant to the temperatures we use in kilnforming that “grabs” the glass and breaks it into shards from that point as the glass cools.

 It is not the schedule that causes the breaks. It is in the shelf preparation.

 The shelf should be cleaned of all the kiln wash and lightly sanded down to smooth. It should then be coated with four thin layers of kiln wash painted in a different direction for each layer. No drying is necessary or even advisable. All kiln furniture must be completely coated with kiln wash.

 If you are re-using a shelf, it must be swept clean before any glass is laid on it.

 Crazing results from the glass sticking to the surface it is resting on.

 

Some additional information:

https://glasstips.blogspot.com/2019/05/kiln-cleanliness.html

https://glasstips.blogspot.com/2020/07/crazing.html