Common Defects in Ceramics and Glassware: Defect Library

Key takeaways

Fired clay and annealed glass cannot be reworked. A cracked or crazed piece is sorted out, so the inspection is a check on the factory's sorting.
A chip or splinter on a drinking rim, loose shards in a box and a cracked wall-hung pan are critical defects.
Glaze or decoration on a food surface must match the tested sample. A changed glaze or decal supplier makes the old test report worthless.
Hidden cracks are found by tapping (a sound piece rings), by light, and with ink or water on sanitaryware.

How to use this defect library

Each table lists a defect, where it shows, how an inspector finds it at the factory, the class it is usually given and how the factory can prevent it. The tables are grouped by where the defect starts: the body and firing, the glaze and decoration, the glass itself, and fit, function and marking.

A critical defect can injure the user or breaks a legal rule. A major defect is likely to cause a return. A minor defect is visible but does not stop the sale. Your defect list decides the final class. Sample sizes follow ISO 2859-1; for tiles the sampling unit is often the carton. See AQL sampling explained and ceramics and glass inspection.

Body and firing defects

Defect	Where it shows	How it is found	Usual class	How to prevent it
Crack near fixing points	Wall-hung pans and basins in sanitaryware	Ink or water test; visual	Critical	Controlled drying and firing; sort after the kiln
Fire crack in a hidden area	Trapway, under the rim of toilets	Ink or water test	Critical or major, as the buyer sets	Even wall thickness; slow cooling
Hairline crack in the body	Mugs, bowls, tiles, planters	Tap test: a sound piece rings, a cracked one sounds dull	Major	Even drying; sort by ring at the factory
Crack at the handle root	Mugs and jugs in stoneware	Close visual; handle flexed gently	Major, critical if it can come off	Attach handles at the right moisture
Underfired, porous body	Terracotta and planters	Dull ring; water seepage test	Major	Full firing temperature and time
Warpage; plate or pan rocks	Plates, large tiles, pan feet	Rocked on a flat surface; flatness gauge on tiles	Major	Even kiln temperature; correct setters

Glaze and decoration defects

Defect	Where it shows	How it is found	Usual class	How to prevent it
Glaze or decoration on a food surface not covered by a passing leach test	Dinnerware, terracotta, enamelware	Glaze code, decal and supplier checked against the test report	Critical	Retest after any glaze, colour or decal change
Crazing	Eating surfaces, wall tiles, frost-resistant planters	Visual at a low angle; ink wiped on where agreed	Major	Match glaze and body expansion
Pinholes or crawling	Glazed faces	Visual against the limit sample	Major or minor by size and place	Clean, dust-free body before glazing
Reactive glaze off the limit sample	Stoneware dinnerware	Compared with light and dark limit samples	Minor or major, as agreed	Control dip thickness and kiln position
Décor tile does not match plain tile	Tile sets	Laid out side by side	Major	Print and fire sets together
Print banding or print-head line	Inkjet tiles	Tiles laid out and viewed from standing height	Minor or major by visibility	Nozzle checks during the run

Lead and cadmium release is a lab result. US limits come from FDA guidance, and Canada sets its own; see lead and cadmium in ceramics and EU food contact rules.

Glass defects

Defect	Where it shows	How it is found	Usual class	How to prevent it
Chip or splinter on a drinking rim	Drinking glasses	Rim checked by light and touch	Critical	Rim finishing; careful handling and packing
Crack in a thermal shock screen	Glasses and jugs	Hot and cold water screen where agreed	Critical	Proper annealing; even wall thickness
Strong strain under the polariscope	Thick bases, joins	Polariscope	Major, critical if pieces crack	Correct lehr speed and temperature
Decoration on the lip-and-rim area without a passing test	Drinking rim area of glasses	Decoration and supplier checked against the test report	Critical	Test each colour and decal supplier
Stone in the wall	Any glass	Visual against light	Major	Clean batch and melting
Sharp pontil mark or unground edge	Base of hand-blown décor	Hand run on the base	Major, critical where it can cut	Grind bases after finishing
Coating peels or rubs off	Lustre, metallic and frosted finishes	Rub test; tape test where agreed	Major	Clean glass; cure the coating
Seeds, small cords, slight tint	Glass body	Visual against the limit sample	Minor	Fining in the furnace

Fit, function, marking and packing

Defect	Where it shows	How it is found	Usual class	How to prevent it
Loose shards or broken pieces in a box or crate	Retail boxes, slab crates	Cartons opened from across the lot	Critical	Sort breakage before packing; brace crates
Food-looking décor piece without a not-for-food mark	Bowls and vases for the US and Canada	Base mark and label checked	Critical or major, as the buyer sets	Fire the mark on; approve artwork
Mixed shade or calibre in a carton or pallet	Vitrified tiles	Tiles from several cartons laid out together	Major	Sort and mark shade and calibre per lot
Fixing holes do not match the template	Sanitaryware	Measured against the template	Major	Control shrinkage; check moulds
Fails rim wash or bulk removal on the rig	Toilets	Flush test	Major	Clear rim channels; set the valve
Leak in a vase sold for fresh flowers	Vases	Filled with water and left to stand	Major	Sort cracked pieces; seal porous bodies

Most claims on ceramics and glass are for breakage. Check dividers, wraps and carton strength, and run a carton drop test where your protocol asks for one. For bathroom pieces, see ceramic bathroom accessories.

Where to catch these defects

Because firing locks faults in, the best moment to act is the first kiln load of a new glaze, decal or glass colour. A during-production inspection at that point checks glaze, warpage and annealing while the rest of the order can still change, and is when samples should be pulled and sealed for a leach test through lab testing coordination. The pre-shipment inspection then checks sorting and packing.

Major centres include Morbi for tiles and sanitaryware, Firozabad for glass and Khurja for pottery; see also Morbi. These are outside our inspector base cities, so travel is charged at actual cost; see pricing.

Sources

Frequently asked questions

Is crazing a defect?

On an eating surface, a tile or a frost-resistant planter, crazing is usually classed major, because cracks in the glaze can hold dirt and let water into the body. On a décor piece sold with a deliberate crackle glaze, it is the design and not a defect.

How do inspectors find hidden cracks in ceramics?

By tapping each sampled piece: a sound piece rings, a cracked one sounds dull. Sanitaryware is also checked with ink or water, which runs into cracks and shows them. Glass is checked against light and, for strain, with a polariscope.

Can an inspector test glaze for lead at the factory?

No. Lead and cadmium release is measured by an accredited lab. The inspector checks that the glaze, colours and decals on the goods match the tested sample, and pulls and seals pieces for the lab.

What is the most serious defect on drinking glasses?

A chip or splinter on the drinking rim, which is critical. Decoration in the lip-and-rim area without a passing test and cracks in a thermal shock screen are also critical.

Why do tiles from one order look different on the floor?

Tiles are sorted into shade and calibre lots after firing. When lots are mixed in a carton or pallet, they do not lay together. The inspector lays out tiles from several cartons side by side to find it.

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Common defects in ceramics and glassware