Wet shelves

 "Was the shelf completely dry? I’ve had pieces practically crumble from a wet mold or shelf."

There is a lot of speculation about wet shelves causing problems. And not just this one. The reported problems centre around large bubbles and glass sticking to the shelf. Generally, the dampness is the result of applying kiln wash. Although the mould or shelf can be damp for other reasons too.

Kiln Wash

I assure you that kiln wash is dry long before the glass sticks together. It is dry before the glass forms a seal to the kiln shelf or mould. The moisture has sufficient time and space to move from under the glass during moderate first ramp rates.

There is a precaution about wet shelves and moulds, though. You need to be careful in placing glass on top of wet kiln wash. It is possible to scrape kiln wash off areas of the shelf when placing the glass. So, the glass must be placed directly onto the supporting surface without any subsequent movement.

Wet Moulds and Shelves

However, if it is the mould or shelf which is wet, rather than just the dampness from kiln wash, different considerations apply.

If a mould is wet, it will need days of air - and then careful kiln - drying before using. It is best to avoid getting shelves and moulds wet. Washing or soaking of these items is not recommended.

The difficulties relate to the nature of wet porous structures. Not only is there free water in the structure of the mould/shelf, but there is also chemical water. Free water is what makes things feel or look wet. Chemically bound water is molecules of water lightly bonded to molecules of the structure. An item can appear to be dry and still contain this chemically bound water.

Both need careful removal. Air drying for up to a week is good for removing the free water. If you do not want to wait that long, you can kiln dry. But this needs to be done carefully. A slow ramp to just under the boiling temperature of water is required to allow the water to evaporate without creating steam. This rate should be less than 100˚C/180˚F per hour. The length of the soak needs to be related to the size of the piece and how wet it is. But one hour is a minimum.

Then another slow ramp needs to follow to remove the chemically bound water. This temperature is around 250˚C/480˚F. Hold that temperature until no fogging of a mirror or glass held above the open port occurs. This will ensure the mould is completely dry and free of the chemically bound water too.

Conclusion

The best advice is to avoid wetting shelves or moulds. It takes a lot of care and time to get them completely dry. The dampness created by applying kiln wash is easy and quick to remove. It can be done during a firing with a moderately slow rise in temperature to 250˚C/480˚F or beyond.