Caring for your ceramics - cleaning ceramics

Cleaning glazed earthenware, glazed stoneware

Glazed earthenware, glazed stoneware, soft paste porcelain and bone china

Although glaze will provide a barrier to liquids, earthenware, soft paste porcelain and bone china are porous and glaze imperfections are common but may not be obvious, so take a cautious approach to cleaning.

Method

1. Use a cotton wool swab (on a cocktail or bamboo stick) dampened with warm water that has had a little detergent added (about one drop of detergent per litre of water) 

2. Roll the swab lightly over the surface, don't wipe or scrub. Make sure your swab is always clean - change it if you see any dirt on it. Using a dirty swab will leave dirt behind in the cracks

3. When the surface is clean, wipe over again with water (to remove any detergent residues)

4. Blot dry with a paper towel

Black basalt vase, about 1785. Museum no. 1506-1855

Unglazed earthenware, unglazed stoneware and biscuit porcelain

Using any liquid to clean unglazed earthenware risks dirt being absorbed into the body and causing stains.  Although unglazed stoneware is more resistant to staining than earthenware, some fine stonewares can mark easily, for example red stoneware and Wedgwood black basalt are prone to watermarks.

Method

To remove dust, brush lightly over the surface using a soft bristle brush (e.g. an artist's sable brush). It is a good idea to put tape over the ferrule (the metal band between the hairs and the handle) to make sure you don't accidentally scratch the surface. Avoid cotton wool because the fibres can snag on a rough unglazed surface.

Pair of figurines, porcelain

Stains

Low fired wares and soft-paste porcelain are prone to staining because of the inherent porosity of the ceramic body. Unglazed areas are particularly vulnerable, but stains can also occur in glazed ceramics if the glaze is cracked or damaged. Stains can be drawn into the body of the ceramic after contact with liquids, foodstuffs, metals or even from materials used to clean or repair objects.

Do not attempt to remove stains with water, solvents or proprietary cleaning products as these can cause the stain to spread, making it worse.

Red stoneware teapot with unfired gold decoration, 1690-8. Museum no. C.4&A-1932

Unfired decoration

This describes painted or metal-leaf decoration that has not been hardened by firing in a kiln and is therefore very vulnerable to damage from handling and cleaning. You can clean around the decoration carefully but cleaning the unfired decoration itself should be left to a conservator.

Floor tile, A.W.N. Pugin

Victorian floor tiles

Floor tiles are common in the hallways and paths of Victorian and Edwardian houses.
These are usually of two different types described as encaustic tiles or quarry tiles. Encaustic tiles are decorated with coloured clay inlays. The leather-hard clay was stamped with an impression of the design, which was then filled with a contrasting colour liquid clay 'slip'. Once dried, the excess fill was scraped away to reveal the design. Encaustic tiles may be glazed or unglazed. Quarry tiles are unglazed and often laid in a geometric pattern of contrasting colours.

Unglazed tiles have a matt surface that tends to hold the dirt. They have often accumulated a variety of floor polishes, wax, varnish, paint or lino adhesive. Over time, the rich bright colours on the tiles become obscured by such layers.

Method

1. Remove loose dirt and dust by brushing or vacuuming.

2. Use a scalpel to carefully remove hard deposits, paint spots or very thick varnish layers. Take care not to scratch the tile with a scalpel - you will need to judge the safest angle and pressure required. 

 3. Add a little detergent (one drop of detergent per litre of water) to bowl of warm water. Use a stencil brush or nail brush with natural bristles to apply the water to a small area about 10 sq cm. Using a circular motion, blot the surface with a paper towel to remove the dirt. Repeat this process, then blot dry before moving on to the next area.

It may be possible to remove waxy deposits with solvents such as white spirit, methylated spirits or a 50:50 mixture of white spirit and water with a drop or two of detergent. Use the solvent on a small swab and apply to an area about 2 sq cm at a time. Work on one tile at a time and blot clean and dry before you move on. It may seem a bit fussy to work on such a small scale in a large tiled area, but it allows you to limit your exposure to solvent fumes and work in a controllable way. Although it is often easier and quicker to use more aggressive methods, these may damage the tiles, particularly if used repeatedly over the long life of the floor.

Proprietary paint stripper can be used to remove paint or varnish. Test a small area first and follow the manufacturer's guidelines for safe usage. This should also be applied following a controlled approach as described above.

As a general rule it is best to avoid applying a sealant because they don't allow tiles to 'breathe', thus trapping damp below and encouraging mould growth. A build-up of silicone sealant can be almost impossible to remove. For housekeeping, use a mat at the doorway and vacuum or sweep regularly.

Glazed earthenware tile, Mexico. Salts are visible as long white needles around the unglazed edges

Salts

Porous ceramics may occasionally contain water-soluble salts absorbed from various sources. In dry conditions, the salts are visible as loose white crystalling powder.

Salts can occur in low fired porous ceramics such as:

• architectural ceramics, as a result of contact with cement, plaster and other building materials
• archaeological ceramics, as a result of contact with soil or salty water
• ceramics used to store foodstuffs

In some cases, salts were part of the original constituents of the clay body itself. In others, materials used for repair in the past, e.g. plaster, or inappropriate cleaning materials, may react and cause salts to form in the ceramic.

Repeated cycles of changing humidity will cause the salts within the pores of the ceramic to alter their state from solution (in damp conditions) to crystals (when dry). As the crystals tend to take up more space in the pores of the ceramic than when in solution, the pressure this exerts can disrupt the ceramic causing the glaze to flake, or the body to crumble.

Surface salts, visible as a white crystalline 'powder' can usually be gently brushed away with a soft brush, though they may reappear (and cause damage) if relative humidity fluctuates. Relative humidity measures the amount of moisture that is in the air relative to the maximum amount of moisture that the air could hold at that temperature.

Conservators use poulticing or immersion techniques for removing salts from badly affected pieces. An object with extensive salts or a flaking / crumbling surface should be referred to a ceramics conservator.