Ceramic Ware Restoration
We are concerned with the repair and restoration of ceramic ware.
OVERVIEW:
This section gives a broad survey of ceramic wares and how they restored. Details of actual working practice, work environment, tools, materials, procurement, marketing, documentation / computer systems, and tax considerations will follow in due course.
MANUFACTURE of CERAMICS:
For a definition and all background knowledge, see Wikipaedia, ceramic, pottery, porcelain, bone china, stoneware, terracotta
All these are based on aluminium silicate, which is an extremely inert substance.
Strongly alkaline substances, e.g. caustic soda, caustic potash and their carbonates, will slowly damage ceramics, and so will hydrofluoric acid. Nothing much else will, other than impact or other mechanical damage.
In consequence, undamaged antique items which have been cleaned up, are 'as good as new' - they do not age perceptibly.
China clay (Kaolin) is the purest natural form. Clay is moulded as desired in a water mixture, allowed to dry and fired at temperatures up to 1400C to form the final object.
The hotter the firing, the harder and denser the final product.
The material shrinks by about 30% during the first firing. This can leave STRESS in the material after cooling.
After the first firing, the object is generally clay-coloured, with a matt finish, and porous. This form is called 'bisque'. The hotter the firing, the less porous the product.
Porous vessels do not hold liquids for long, and glazing was devised a very long time ago, to overcome this problem. This is a layer of a special glass applied to the surface, which gives the pottery its high shine and make it impermeable.
Glazes are usually clear, but may be coloured for decoration. For application techniques see Wikipaedia, pottery.
Porcelain is impermeable to start with, but is generally glazed for decoration.
Bisque may also be left as it is - in increasing order of value:
The bisque item is more usually finished and decorated by various procedures:
Applying plain or coloured glaze, and firing.
Colouring/painting and then glazing, then firing. This is called under-glaze painting, and gives the highest quality. The paints used are insoluble metal oxides or salts (i.e. inorganic pigments), and the technique is difficult because the true colours are not seen till after firing.
Glazing, firing, then painting and firing again. This is called over-glaze or on-glaze painting, and uses inorganic pigment. Printed paper transfers were devised for bulk production of detailed patterns. ( You can often tell older transfer prints by looking for clumsy joins)
Glazing, firing, then painting and firing, or curing using at least some organic pigments (I use 'organic' in the chemical sense) and curing at up to 200C in a resin mix - commonly use in transfer prints on inexpensive modern wares - the ones where the pattern scratches off fairly quickly in use..
"Firing" is done in a kiln (furnace), "Curing" is done in an oven - e.g. an ordinary domestic oven.
Painting and firing - no glaze being used; gives a matt finish. (Used by Heubeck and Goebel/Hummel and some other French antiques, and Royal Dux )
Just painting. (used by F Goldscheider)
Each firing must be done at a lower temperature than the one before it.
Special features of the glaze:
Crazing. Up to about 1900, glazed pottery was often produced on which the glaze shrunk rather more on cooling than the pottery substrate. When it cooled, the glaze developed a network of tiny cracks. Potters tried to avoid this obviously, but the crazing most generally happened after the pot was sold, when it was in day-to-day use. Cheaper modern tableware will often develop larger patterns of glaze cracks.
Crackle glaze. The Japanese did crazing on purpose to make a decorative finish, and sometimes treated the surface with black iron oxide to emphasise the finish. (See Wikipedia, Satsuma ware). It's called 'crackle' to show it's not a mistake.
Tin glaze. Up to the end of the 18th century, European potters could not match the fine blue-on-white porcelain imported from the Far East. They used instead a bluish-white opaque glaze incorporating tin oxide, and often decorated with cobalt blue. (Wikipedia, Tin glazed pottery) Looks pretty crude but is higly collectable.
Coloured glazes may be transparent or opaque. Common transparent colours are brown (iron oxide) or blue (cobalt blue). The popular bone-china horses made by Beswick ( now incorporated into Doulton) use transparent brown glazes on a lighter brown ground, applied with an airbrush, to produce their lifelike decoration. Moorcroft ware gets its unique character from coloured glazes, applied (I suspect) by finger painting.
Gilding - the bane of the restorer - is done after all other decoration is completed, and usually on a glazed surface. The original gilding generally involves a light firing (See Wikipedia, Gilding Ceramics ) and the finished product is usually a very thin shiny layer of metallic gold.
Early processes often gave duller finishes, and gilders' secrets were closely guarded.
Gilding can be done on a bisque, or on a matt finish glaze, but it comes off rather too easily. Royal Worcester was inclined to do this at one time.
GLOSSARY:
Firing crack: Antique items will sometimes have firing cracks which often have black marks adjacent to a flaw in the glaze caused in manufacture. These cannot be cleaned, and should be discussed with the customer - whether to leave it or cover it up.
REPAIR & RESTORATION
NOTE: the tools and materials described in the following notes are (mostly) produced for other purposes, and have to be acquired from a variety of suppliers. There are very few who make any sort of specialty of it. Full details will be given later.
REPAIR in this context means, to mend a broken item.
It is NOT FEASIBLE to repair ceramics by re-firing. Attempts have been made; the results look terrible.
You have to use a material with a lower fusion temperature than the original, in the form of a wet powder or 'frit' - so it is not a neat join
Any secondary firings for glazes and colours are usually damaged by this.
If it all goes wrong (and it generally did) there is no way to reverse the process.
RESTORATION generally means (a) re-modelling missing bits (b) redecoration, so you can't see the joins.
TABLEWARE should be restored ONLY for ORNAMENTAL purposes or very occasional use, and preferably not to be in contact with any food or drink - you cannot know what toxicity or allergy will emerge. I have also determined that no paint or adhesive which I have ever used is resistant to repeated washing, although some lasted a good while. Any customer should be warned.
STAGES:
Note: I shall be using this section as an index, via internal hyperlinks, to detailed descriptions of how to do the work described. Stuff is to be added where I mark it ******************
Dismantling and Cleaning. A lot of the stuff you get in will have been stuck together by amateurs, e.g. the owner's husband, with unfortunate results. Other items, especially those from the antique trade, will have been restored previously; this is not always so obvious.
Dismantling:
The majority of glues can be softened with boiling water, and the item taken apart gently. With luck, a lot of the glue will become unstuck and peel off.
The rest, mainly setting resins, i.e. epoxy (Araldite) or cyanoacrylate (Superglue), can be dissolved with paint stripper (Nitromors). Gilding survives this treatment, but any painting from a previous restoration will be removed too.
If you get a stapled antique (see below) the staples - which generally do not penetrate to the other side - may be removed with fine pliers after one of the above procedures.
Cleaning:
Practically everything you get in needs washed before any repair work is done. Ornaments sitting out in the open for a long time can get amazingly filthy. A dishwasher is best - it was designed for this job! (except only for F. Goldscheider - which is painted - and some gilded Royal Worcester on bisque ). A dish wash at 50C using Sainsburys Basics dishwasher tablet works fine for most things. Any dismantling or bleaching needs to be finished by a wash.
If washing doesn't clean it, bleaching may be needed particularly for items with dirty cracks; or antique table- or kitchen-ware with a crazed glaze, through which the pottery underneath has got stained. Bleaching is done at room temperature with either hydrogen peroxide or chlorine from sodium hypochlorite (a.k.a. thin bleach).
Peroxide may be used safely indoors.
Chlorine bleaching, as I shall describe it, is best done outdoors.
Rust stains - identified by their rusty colour, and surviving any washing and bleaching, - can be removed with phosphoric acid
A textile cleaning gun - which delivers a pulsed jet of high-pressure water - is useful for getting particulate dirt out of cracks, and particles of old glue or paint off an opened join.
After any washing with water, allow the item to dry overnight before using any adhesive - particularly porous materials, which can soak up a lot at the broken edge. If in doubt , or the workroom is cold, leave it longer or dry off at about 50C in an oven.
Assembly: Ceramic ware can only be repaired by:
Mechanical means - up to about 100 years ago, until better adhesives were invented, broken items were sometimes stapled - see illustration below. You may get some of these to be dismantled and restored with ordinary adhesives! (The staples are only on one side, stuck in with shellac or plaster)
Stapled join
Use of adhesives. Modern adhesives are used to make joins which are as mechanically strong or stronger than the ceramic material, but which are inconspicuous and can be taken apart again and re-done if need be.
Epoxy resins are the most useful. These are two-part adhesives which, when mixed, set at room temperature to give a tough, insoluble join. If this is done properly, the join will be as strong as the original ceramic at room temperature. Most (but not all) epoxies tend to go yellow with time and/or heat, which you need to bear in mind if the join is visible.
Cyanoacrylates (Superglue) have limited application because of their poor resistance to impact, but are indispensable for the very fine close work.
Solvent adhesives should not be used, except for Paraloid, which is approved for certain museum work. It is ethyl metacrylate, which dissolves in acetone, and is a lousy adhesive (in my opinion), but it does not yellow, is easy to remove, and does not discolour earthenware under the glaze. If the thing is just going to sit in a glass case, sure the adhesion doesn't need to be great.
Before using adhesives you need to clearly classify your work piece as being porous (earthenware, pottery) or non-porous (stoneware, fine bone china, porcelain). With experience, you can see the difference. If in doubt, apply a drop of acetone to the broken exposed. surface - a porous surface will soak it up quickly, but it will sit on a non-porous surface till it evaporates - e.g. about 20 seconds.
The reason for this is simple. You will get the best join with the thinnest adhesive you can use, but a reasonably thin liquid (many cyanoacrylates, Araldite 2020) will soak into porous material and away from the surface to be joined, leaving you a join which appears OK but is in fact very weak.
Joining non-porous ceramics is easiest. Araldite 2020 is the best adhesive to use. **********
Joining porous ceramics. **************
How do you know if a join is strong enough? If is survives your subsequent handling, it is strong enough.
Special techniques are needed for joins with a very small surface - e.g. replacing fingers on figurines, or making new ones, petals on flowers, and suchlike detail.
Use of fillers. You will very rarely get a perfect join, where the two sides match precisely along the whole length. In nearly all cases, one side of the break is splintered, and small or large pieces may be lost entirely. There are two sorts worth considering:
Modelling compounds. By far the best for this job is Superfine White Milliput. This is a two-part epoxy putty, which can be worked and shaped very like Plasticene, or (with a little modification which I shall describe in due course) can be applied with a brush or by spraying. When allowed to set or after light curing, it may be carved or sandpapered to shape.
For cracks which cannot be closed, epoxy adhesives may be used as fillers. This usually requires the use of a thickener.
Fillers may be coloured to (roughly) match the ground colour with powdered pigments, if appropriate.
The use of fillers shades into re-modelling - see below.
When all the adhesives and fillers have been added, it is essential to be able to remove the surplus! On all ceramics - except those which have been finished with unfired paint - you need to smooth the surface down.
A modelling knife can be used to cut adhesive or filler off a join, where it has squeezed out.
Glasspaper can be used to smooth down fillings, without scratching the glaze.
True glasspaper consists of powdered glass stuck on to backing paper. Glass does not scratch the glaze (which is also glass!), so glasspaper can be used freely in this application. It is increasingly rare, being generally replaced by harder-wearing abrasive papers for general use - although these modern improvements are also often described as glasspaper!
Abrasive cloths are useful for finishing off and polishing the smoothing process.
Wet-and-dry (silicon carbide) paper will abrade ceramics, and may be used to smooth a bad join, if all else fails.
Bad joins can happen when the stress left from firing (see above) resolves into a strain when the item breaks. Naturally, things break most easily where they already stressed. This is described as 'sprung'; the two sides of the break no longer fit together. (Bad joins can also occur when the break has been badly joined, in this case you take it apart and try again....)
Remodelling *************** to be added
Decoration ************ to be added
WORKROOM, TOOLS, MATERIALS
The Workroom
Because of the smell of the variety of solvents and chemicals used it should be separate from living quarters. It should have good ventilation, and daylight for colour matching - a window which you can open . Shelves/cupboards for materials storage, also space for packing materials. A workbench and work cabinet near the window. I have found that an old bed makes a good place to keep work in progress, and folding storage crates (Homebase) useful for storing pending items. A typist's chair is ideal. (Try second-hand office furniture suppliers)
INFLAMMABLE SOLVENTS are best stored away from any living space, e.g. in an external garage, except in modest quantities needed for workroom use. (e.g. White Spirit, Xylene, Acetone)
CHILDREN should be kept out of the workroom at all times.
Work wear
Carpenter's or chef's apron is useful, pockets, and the lap often catches any bits you may drop.
Goggles or glasses. You need some eye protection; I have found gallses are sufficient. Glasses which give magnification are good for close work - reading glasses are inexpensive. I use the strongest off-the-shelf type (3.5) plus a flippable attachment http://www.goodideas.uk.com/search/details/name=Clip-on+Magnifiers/ref=234 These glasses get fouled up especially when air brushing and should be regarded as semi-disposable. Do not use prescription spectacles, they cost too much!!
Domestic rubber gloves, medium to heavy duty, e.g. Marigold brand. Used for dismantling work with boiling water or Nitromors, and chlorine bleach.
Disposable vinyl gloves, as used for hair colouring. (Sally Beauty Supplies, 32/33 Southbridge, Edinburgh - also get you peroxide here) - pack of 100. Used for: Working with cyanoacrylates (superglue) - less painful than finger skin - and peroxide bleaching.
Tools & Equipment
Dishwasher - very useful but not essential. Kitchen sink, kettle.
Domestic oven ( preferably electric ) size e.g. 17" wide * 15" * 15" overall inside, which can be set to 80C, is essential. Can also be used for cooking!! On no account attempt to use a microwave oven, it will break things up even more.
(These items do not need to be in the workroom - the kitchen equipment will do very well)
Baking trays ( to hold stuff in oven)
Work Cabinet. This has two main purposes:
To provide a consistent strong light to work in, and to help in colour matching.
To provide some extraction of solvent/paint vapour when airbrushing.
Compressor and airbrush(es)
Textile Cleaning Gun I got mine from the supplier below, who is in the USA and deals with mineralogical supplies. Can't find a simple supplier in the UK. They all seem to be made in the far east, and websites are wholesale only. Perhaps ask the manager at the nearest dry cleaning store. The one I got is quite good but the advertised pressure control doesn't do much. http://stores.ebay.com/BOWERS-TRINITITE-COLLECTION
Just use cold water in it. It can damage very soft pottery (e.g. Hummell figures, F. Goldscheider) or loose glazes (crackle finish). If the pressure control doesn't work, just hold it further away. Don't put you finger in the jet; it can take the skin off.
*********************** lots more
Materials & Consumables
Nitromors : I have always used Nitromors Paint & Varnish Remover (Brown tin) or Nitromors Master Craftsman's Paint and varnish remover (Yellow tin) - a bit thinner than the brown tin. There are all based on methylene chloride and there is no reason why others should not work just as well. Available from Homebase or any decorator supplies. Always wear gloves and goggles when using this stuff. If you get it on your skin, it will do no real harm, but it will feel as if it did.
Kitchen Rolls : Sainsburys Basics - best value. Do not get any which have pretty prints on colours on them. Use as disposable wipes for everything.
Beakers Sainsburys Basic plastic beakers, 99p for 50
Cocktail Sticks In Sainsburys among the paper plates etc.
Hydrogen Peroxide Sally Beauty Supplies, 32/33 Southbridge, Edinburgh: Liquid Peroxide 12% 40vol, 1 litre (much better value than you will get from a chemist, who only has small bottles of 20 vol.)
Ammonia Starpax household ammonia solution (9.5% w/w) : from any decent chemist.
Pipettes, 1ml (plastic) From laboratory supplier or Sylmasta
Fumed Silica (filler grade) Cabosil Fumed Silica from Tomps. 250 grams doesn't sound like much but it is extremely light and occupies a volume of several litres. Try not to breathe it in. It is totally inert but irritating.
************* lots more here too
PROCEDURES
Dismantling with Boiling Water.
Suitable for: all, especially very old joins, clear bubbly glue, brownish glue. This works for maybe 3 items out of 4.
Kit: Kettle, blunt knife, tweezers, rubber gloves, kitchen sink, cloths, modelling knife, magnifier.
Boil up a large kettle. Place the item on a cloth on the draining board of the kitchen sink ( cloth prevents scratching the sink ) and put the plug in the sink, in case small bits get washed away. Wearing a pair of rubber gloves, hold the item so that the water can be poured over the join. Pour as slowly as you can! If the join does not show any sign of softening after a minute, apply gentle pressure. If it fails to open, DO NOT FORCE IT, as you may break a new bit off instead! When the join opens, scrape off as much glue as you can with the knife or pull it off with tweezers ( sometimes it just peels off in strips )
Dry off the pieces and remove any residual glue. Every last particle must be cleaned off; a modelling knife and a magnifying glass can be useful. You can see (and feel) that the join is clean, if it fits together snugly. If all else fails, treat the edges with paint stripper.
Dismantling with Paint Stripper
Suitable for: everything else, other than items which have been painted!
Kit: Protective clothing, i.e. kitchen gloves, apron and some eye protection. A selection of tins or containers which can be closed - e.g. biscuit tins, coffee tins. ( polythene and polypropylene also seem to be resistant ). Plain polythene bags to wrap large objects. Masking tape. A couple of long artist's brushes, 1/4 to 1/2" or so. An old toothbrush or two. Nitromors or other paint stripper. Tweezers or long-nosed pliers to pick bits out of the stripper. Methylated Spirit in a handy jar. A work surface which will not be damaged by paint stripper. Paper kitchen Roll. Plastic bucket or bowl, Fairy liquid or similar.
Put on the protective clothing. (It won't actually do your skin any harm, it'll just feel like it.)
Use the paint brush to apply stripper to the join. Put the item in the smallest container you have which will hold it. Pour in 10 - 20 mls stripper into the bottom of the container - this prevents the join from drying out. Seal up the lid with masking tape - the vapour pressure is quite high!
If the item is too large to go in a tin, apply the stripper and put a polythene bag over it. If possible, lodge it so that the join is low down, and add extra stripper to the bag, and seal it all up as well as you can with masking tape.
If you are dismantling something that you have just glued but got wrong, it should break up in minutes. Otherwise, give it several hours or overnight. The item should have fallen apart without any force applied. Pick out the pieces, put them on a bit of kitchen roll and scrub them with a toothbrush and methylated spirit. Count and note the number of pieces you have!. When all are out and cleaned with meths, wash them in warm water and dish washing liquid; (or in the dishwasher if large enough) rinse and place on a cloth to dry out.
Suitable for all items.
Bleaching is effective only on items on which the glaze has been damaged or is absent, that is: on cracks, crazed or crackle glazes, and bisque ware. An intact glaze will keep dirt out, unconditionally. Do not attempt to clean firing cracks.
Kit: Small beaker, 1 ml pipette, disposable vinyl gloves, goggles or glasses, spatula or small spoon, hydrogen peroxide, ammonia, Fairy liquid, fumed silica (filler grade), brush (the sort of small wide brush that ladies use to do their hair roots with is ideal for this), cocktail stick.
Put on the vinyl gloves and the goggles. Peroxide will REALLY damage your skin, when the ammonia has been added.
For a dirty crack: Pour about 20ml into a beaker and add about 1 ml ammonia with the pipette. (this activates the peroxide). Add the tiniest amount of Fairy liquid you can get on the end of a cocktail stick as a wetting agent. Add about twice the volume of fumed silica to the mix, till it is thick enough to stay where it is put. Use the brush to apply gobs of the mix along the line of the crack. Put the item on a waterproof surface; do not cover the peroxide mix. Let it stand for a day or two till it dries out and the silica falls off. (Don't be tempted to poke it with your finger to see if it's dry -use a glove!) Brush off the excess and dish wash again. If the dirt still shows, but has faded, repeat the process until clean or no further improvement is obtained. At this point, try the textile cleaning gun. (use water only!) This is very good for particulate dirt, but beware of causing slight damage on soft pottery.
For a dirty area, for example under a crazed glaze : same as above, but spread the peroxide mix over the whole area affected. (Don't use the textile cleaning gun at any point on a crazed glaze - it can wash the glaze off if applied too hard.) If the item looks nice and clean after a wash, leave it for a few days to dry out completely. You may find that a dirty reddish brown colour comes back again. This is a mobile water-soluble stain, probably caramel, and can be very hard to get rid of completely. Repeat peroxide/wash/dry cycle till fed up. IF ALL ELSE FAILS, try the chlorine bleach.
I have found this to be of use only for antique kitchen- and tableware which has been grossly stained in use, on a crazed glaze, such as jugs, drinking vessels, and carving dishes, and which have not responded to other means of cleaning. In such cases, the stain has penetrated deep under the glaze, and sometimes migrated to parts where the glaze is sound. This means that the bleaching agent needs to soak in through the crazing and penetrate to every part under the glaze. This needs a stable bleaching agent and peroxide decomposes within a few days after ammonia us added.
The problems with chlorine are:
