Ceramics Notes Pdf

[Jen Ondrey]

Thisis a detailed list with explanations. I also created a pdf checklist that you can download, print and keep handy in the studio. The link to download it is at the top and bottom of this article.

If you love learning about and discussing glazes, I'd like to invite you to my free social learning Facebook group called Understanding Glazes with Sue. The group is full of videos and discussions about firing, mixing glazes and fixing various glaze issues. Please join!

Just found your post. Great list of things to think about/log. Finally have my own studio space. No going to the instructor with questions, so the internet is now my teacher. Looking forward to learning more. Thanks for sharing your experiences with clay.

I put a consecutive number on the bottom of each piece using iron oxide or black underglaze and then wax over it when I prepare the bottom before glazing. Then my glazing log book has each piece listed with notes next to it under the date it was glazed. This is great for forever identifying each piece. When it goes to a gallery, I can use it as an inventory number. If someone buys a piece and asks me years later for a replacement,, I can look up how it was glazed and come up with something very similar. If a glaze combination comes out great or terrible, I make a note in red next to the glaze log entry.

Many of the problems encountered with ceramics and glass can be avoided through proper handling and storage. This Note provides guidelines on the care of ceramic and glass objects in museum collections.

Ceramic objects are as varied as the cultures that produced them, in terms of their construction, composition, style, and use. Generally, however, they have a clay body, often with a glaze applied to the surface. The body (also known as the fabric) consists of clay minerals, temper, and impurities. Temper can be any particulate material used as a filler that helps reduce shrinkage and allows gases to escape when the object is fired. Tempers include sand, shell, chalk, mica, and ground-up fired ceramics. Organic material, which sometimes burns away during firing, can also be used as a temper.

Pure clays are white; it is the impurities associated with clay minerals that cause their particular colour. Iron, the most common colouring compound found in clays, imparts red, grey, and buff tones to the ceramic. The colour of the ceramic fabric is the result of the impurities present in the clay and the conditions under which the object was fired.

Glazes are sometimes applied to decorate or waterproof an object. A glaze is a layer of glass fused to the clay fabric. To prevent the glaze from cracking when the ceramic is fired, the thermal expansion and contraction properties of the glaze should match those of the clay fabric.

Glazes are composed of silica and fluxes such as sodium, potassium, calcium, and lead. Colorants, mostly metal oxides, can also be added. Fluxes lower the melting point of silica and can change the physical properties of glazes, such as the degree of hardness and resistance to chemicals. Silica is either derived from the body of the ceramic or added separately. For example, in forming a "salt" glaze, salt (sodium chloride) is added to the kiln as the ceramic is fired. The salt vaporizes at a high temperature and the sodium combines with the silica in the ceramic body, forming a characteristic thin, orange-peel textured glaze. The chloride goes off as a gas into the atmosphere.

Ash, which contains the fluxes sodium and potassium, can be applied directly to a ceramic fabric to create a glaze. Oriental stoneware vessels were often decorated with ash glazes. Like a salt glaze, ash glazes form at a very high temperature.

Lead glazes can be created in three ways: by applying lead oxide directly to the surface of the ceramic, by mixing it with silicates in a slurry, or as a frit. "Frits" are glazes that are made by melting silica with fluxes to form glass which is then ground to a fine powder and applied to the ceramic. Lead glazes have a lower melting temperature than ash or salt glazes, and are the most common glaze on pre-19th-century European ceramics. If the lead was applied in a frit that was not properly made, chances are high that the glaze will have been damaged if the vessel was used to hold acidic liquids, such as orange juice, vinegar, or wine.

A variation of the pure lead glaze is the opaque white glaze, known as majolica, delft, or faience, which is produced by adding tin oxide to a lead glaze. This addition lowers the melting point of the glaze even more.

Ceramics that have been fired at high temperatures generally have a hard, glassy fabric and a securely attached glaze. Ceramics with soft, porous fabric or poorly attached, flaking glaze have usually been fired at lower temperatures, unevenly fired, or, in some cases, may not have been fired at all. These characteristics affect the stability of a ceramic and its ability to tolerate adverse environmental conditions.

Pottery that has not been fired in a kiln, such as some prehistoric North American pottery, can be very soft and crumbly. These objects can be extremely fragile when they are recovered from an archaeological site after spending several hundred years buried in the ground.

Like glazes, glass is composed of silica, fluxes, and colorants. It is an amorphous material, i.e. it lacks the orderly three-dimensional network that characterizes a crystalline solid. Although glass is rigid and hard on cooling, the atoms of glass are randomly arranged like the atoms of a liquid.

The physical and chemical structure of glass helps explain its transparency, brittleness, and characteristic deterioration. The arrangement of the atoms allows light to pass through without interference, and gives glass the transparent quality of a liquid. Brittleness results from a combination of factors: the rigid nature of glass, the internal stresses that are set up in glass as it cools, and surface flaws that concentrate applied stresses. Because glass is a homogeneous mixture with no internal boundaries or discontinuities, cracks in glass tend to spread.

Commonly used raw ingredients for glass include quartz (silica), mixed with sodium ("soda"), potassium ("potash"), or lead as fluxes, and calcium ("lime"), which acts to stabilize the glass. Small amounts of colorants are often used, including iron, copper, cobalt, and manganese. Lead increases the density and improves the optical qualities of glass.

The composition of glass, particularly the proportion of silica to fluxes and stabilizers, determines its stability. Glass should ideally be a homogeneous mixture; inhomogeneity can make the glass more susceptible to deterioration. If the proportions of the ingredients are wrong, the fluxes will react with water and leave a depleted and fragile surface ("Weeping and crizzling" below).

The surface of deteriorated glass is sometimes iridescent. In extreme cases, thin layers of degraded glass resembling onion skin may form on the surface. Glass in this condition is very fragile, and the surface layers can disintegrate easily.

Weeping is characterized by the formation of liquid droplets on the surface of glass, creating a cloudy and disintegrating surface layer. It occurs when water vapour from the atmosphere leaches some of the components from the glass. Fluctuating relative humidity (RH) can make the problem worse. The liquid droplets may be very alkaline and can damage material with which they come in contact (for example, a textile with glass beads sewn onto it; see CCI Notes 6/4 Care of Objects Decorated with Glass Beads).

Crizzled glass has a network of very fine fissures on the surface. In the early stages of crizzling, cracks may be visible only under a microscope. If the condition is more severe, the glass can appear cracked and cloudy, with fragments of glass spalling from the surface.

If a ceramic object has been buried in salty or alkaline soil, submersed in seawater, or used to hold salty material (e.g. chamber pots), its porous body may have soaked up soluble salts. These salts can react to RH changes: dissolving in high humidity and recrystallizing in low humidity. Dissolution and recrystallization can cause a loose glaze to flake off, or the surface on unglazed areas to spall. Keeping the RH constant helps to prevent this type of damage.

The general, common-sense rules for handling museum objects also apply to ceramic and glass artifacts. Unlike other materials, however, ceramics and glass are hard, brittle, and can easily crack or break on impact. Storage and display areas must be designed to prevent unnecessary handling.

Ceramics and glass are generally less sensitive to extremes or fluctuations in humidity, temperature, and light than other materials (e.g. wood, ivory). However, this applies only to objects in good condition, and not to those with any of the problems discussed above. Identifying the early stages of deterioration can be difficult, so treat all pieces as if they are potential problem cases. Protect these objects from extremes or rapid fluctuations of temperature or RH.

Glass that shows signs of active weeping or crizzling should be kept in closed containers where the humidity can be kept as constant as possible. Sudden drops in RH to below 40 % or rises to above 60 % can damage deteriorated glass.

Avoid temperature extremes, particularly rapid temperature changes. This causes expansion and contraction, which may result in breaking or cracking. Never heat glass, or put it in a sunny place or under spotlights where it might become very warm.

Some types of glass are adversely affected by ultraviolet radiation. For example, colourless glass sometimes includes a small amount of a decolourizing agent, such as manganese dioxide. When exposed to ultraviolet radiation, this type of glass will turn purple in a process called solarization. Glass should not be stored or displayed in direct sunlight unless ultraviolet filters are used.

Ceramic and glass objects are easily broken when dropped or jarred. Store these objects so they are protected from physical damage. Also, protect them from dust and grime to reduce the need for cleaning.

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