Crawling is described as the bare area(s) in which the molten glaze does not cover during the glaze firing. The severity of the defects may vary from case to case from the isolated area (pin hole) to more serious cases such as 'beading' which it is akin to forming beads of water on waxed surfaces. Generally, it is caused by surface tension forces (particular in molten glaze) overcoming the adhesion forces.
Underglazes decorations may cause crawling due excess of the medium (in underglaze decoration) or insufficient fluxes.
Excessive mediums cause the greasy surfaces causing glazes unable to stick to the underglaze decoration. Hardening (about 600 C) should burn away the medium and will cause colour to sinter and bonded firmly to the ceramic ware.
Insufficent flux can be remedied by adding flux (borax frit) in 5%-50% or SCMC binders in 1%-3% to colour pigment may help for the glazes to attach to underglaze decorations.
Area may be contaminated with grease, specks of dust or similar materials which will cause lack of adhesion glaze which causing the glaze to roll back during glaze firing
Thick glazes may aggravate the problems since minute cracks may be formed during firing which is then healed leading to the formation of large cracks in molten glaze producing crawling. It is usually seen in inside holloware and around handle joints. Thin the glaze by using water or adding a bit of binder (to increase adhesion)
Consider using spray glazing instead of manual dipping as this has several advantages
# The glaze is partially dried when travelling from nozzle guns to the ceramic surfaces (Reduced water contents will be helping in forming less crawling)
# The sprayed glaze is deposited in a more 'fluffy' arrangement which offers less resistance for water vapour to escape during glaze firing
Overgrinding glazes may produce dusty glazes with poor adhesion. Also due to the chemical compositions, it will become more viscous increases crawling risks
Zircon opacified glazes, zinc oxides, high alumina and leadless glazes are considered as highly viscous glaze. Highly viscous glaze may be corrected using extra additives such as flux (borax flit) or firing at higher temperatures.
Glaze recipes formed partly of colemanite by Asian sources which generate voluminous gases which cause patches of glazes ejected from wares. Adding flux calcium borate frit to the glaze or slower rate firing (increase soaking time) may reduce the crawling to ensure the gasses had ample times to escape.
