Stage Two of Pottery - this step explains why the purpose of glaze and the danger that needs to be protected from.

Remember that you should have a notepad or printout of your worksheet. The worksheet is located at the lower part of your Pottery Screen. If you do not have a printout then take notes so you can put the information needed in the boxes.

What is glazing of Pottery

Glazing pottery is the process of shaping and firing ceramic ware. The drips are called glazes. Glazes are made from a variety of materials, including minerals, metals, and even glass. They are applied to the surface of the pottery before firing, and they interact with the body of the pottery to create a variety of effects.

What is glazing of Pottery viewed by another website

Ceramic glazes are both useful and decorative. Unglazed ceramics are porous. If liquid is left in an unglazed vessel, it will leak out through the open clay pores. Glaze coats ceramic surfaces, making them impermeable and waterproof. Glazes are also an expressive medium for artists.

Glazes are generally applied as liquids to a ceramic surface by painting or dipping an object into a bucket of glaze. Glazes and oxides can be rubbed into ceramics as dry powders.

A process popular in Europe in the 17th and 18th centuries involves tossing salt or sodium bicarbonate into a kiln near the end of the firing process. The salt vaporizes and merges with the surface of the clay, forming an interesting mottled glaze. This process is known as salt or soda glazing. According to CeramicArtsDaily.org, soda firing is the technique most used by potters today, usually in gas-fired kilns.

History of Glazing

Tracing the origins of glaze sheds light on various customs and cultures. According to the History World website, the Greeks developed fired unglazed earthenware for storage and cooking in the 5th century BC. The pots were porous and unable to hold water until the development of glazes in the 1st century BC in Egypt.

The first blue-green glassy glaze is commonly known as Egyptian paste and is still popular today. Glazed ceramics were simultaneously developed in early China and the Middle East, and traded along the Silk Road. Glazed ceramics were coveted commodities and the active exchange of ceramic styles and techniques impacted future development of the craft. Early European pottery and glaze techniques were influenced by eastern ceramics that traveled to Rome.

What Is the Purpose of Glaze in Ceramics? by L. Bradley (ehow.com)

Glaze Application

Glaze is normally applied to bisque fired pots.  Bisque, also called bisque ware or biscuit ware is clay that has been fired one at around 1832F (1000C).  Bisque firing turns raw clay into ceramic pottery, and clay that has been bisque fired is still quite porous. 

Before it has been fired, pottery glaze is applied in liquid form to the bisque ware.  It can be applied in a few different ways.  And the way you glaze your pottery is really a matter of personal preference. 

The main ways of applying glaze to pottery are:

• Painting it on
• Dipping the pottery in a bucket of glaze
• Pouring the glaze over the pots
• Spraying the glaze onto the pottery

When the glaze is applied, the water from the glaze is drawn out of the liquid by the porous bisque.  So, the glaze dries out very quickly.  This leaves a powdery dry coating of the glaze material on the pottery surface. 

Usually, 2 or 3 coats of glaze are applied to build up the right thickness.  However, the number of coats used depends on how the glaze is applied.  Normally more layers are applied when brushing the glaze on the pot.

When the glaze has dried on the pot, the pot is ready for glaze firing.  To understand what happens to glaze when it’s fired, we need to know what the glaze is made of. 

Danger of using Glaze

Like clay, glazes also go through a process of sintering.  This is one of the first stages in the glaze firing.  During the sintering phase, the surface of the glaze particles heats up and becomes soft.  At this stage, the glass-forming material in the glaze hasn’t begun to melt.  However, the glaze particles have begun to stick together. 

What is sintering - Sintering is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. It happens as part of a manufacturing process used with metals, ceramics, plastics, and other materials.

The sintered glaze has lost the powdery flakey quality it has when it first dries. But it hasn’t become hot enough to have melted and formed a glossy surface (source).

As the temperature in the kiln increases, the glaze starts to melt and fuse.  The structure of the crystals in the glaze materials begins to alter and the glaze begins to become molten. 

When the kiln starts to reach its target temperature, the glaze begins to boil.  At this point in the firing process, the glaze particles have fused into a liquid and the glaze will bubble.

During this process, the glaze is undergoing chemical and molecular changes and gasses are being released.  Once the gasses have escaped the bubbling process calms down and the surface of the glaze smooths out.

If the glaze firing ends too soon, gasses that have not had a chance to escape can get trapped in the glaze.  This can cause the glaze to be cloudy or to blister.  Potters will sometimes use a soak or hold period at the end of a glaze fire. 

During a soak, the kiln is held at a target temperature for a brief period.  Usually a few minutes, sometimes longer.  The soak provides time for all the gasses that need to escape to have left the glaze. 

After this process is complete, the kiln is left to cool.  As it cools the glaze forms a hard glassy surface on the pottery.

** Important to remember and this website explains it to you **

Broadly speaking there are three types of clay for pottery.  They are earthenware, stoneware, and porcelain.  These different types of clay have different qualities and are fired at different temperatures.  Earthenware is fired at lower temperatures than stoneware.  And porcelain is usually fired at a higher temperature than stoneware. 

The glaze you use needs to melt at the same temperature you are firing your clay at.  For that reason, glazes are usually categorized as low fire, mid-fire, or high fire.  These categories refer to the temperature range they are suitable for.

What happens during the glaze firing depends to an extent on the type of clay and glaze you are using.  And that is what we will cover in the next section…

Is Pottery Glaze Toxic and Why Is This an Issue?

Once upon a time, Europe loved lead. From painting to face powder to pottery, lead was used in a huge range of artistic endeavors.

Of course, now we look cringe at the idea of lead in makeup.  But originally the long-term dangers to health were not known.  It wasn’t until much later that people discovered and understood the implications of lead poisoning.

We are now much more conscious of the effect of exposure to different chemicals and compounds.  As a result, there has been growing concern over the toxicity of certain glazes.

For a long time, lead-based glazing was common. If putting on some lead face powder could make ladies at court sick, imagine what ingesting it by eating and drinking from lead-glazed plates and cups must’ve done.

Lead as well as cadmium is among the most common toxicity issues when it comes to pottery glazing.

Glazes and Danger

Glazes used to color or finish clay pieces are a mixture of silica, fluxes and colorants.  Common fluxes include lead, barium, lithium, calcium and sodium, and are used to lower the melting point of silica.  The actual colorants, which are an assortment of metal oxides usually account for less than 5% of the glaze by weight.

Originally, soluble raw lead compounds including red lead, white lead, galena, and litharge were used as fluxes in low-fire glazes.  In fact, over 400 cases of lead poisoning were reported in British potters in 1897.  Lead frits and good housekeeping greatly lowered the number of potters that had been poisoned by these highly toxic lead compounds.  Frits are made of melted minerals and metal compounds that are sintered and ground into powder form.  While lead frits are sometimes assumed to be insoluble and nontoxic, leaching tests with acids have shown that many frits are as soluble as raw lead compounds and, in fact, there have been cases of lead poisoning from both inhalation or ingestion of these.

High fire porcelain and stoneware techniques eliminate the need for lead as a flux.  Also, alkali earth or alkaline earth fluxes can be used for low-fire conditions instead of lead. Silica may also be removed from leadless type glazes.  The substitution can be based on boric oxide as the glass-former, instead of silica.  Alkali earth fluxes include sodium, potassium, and lithium oxides; alkaline earth fluxes include calcium, magnesium, barium, and strontium oxides.  Minerals containing these fluxes include certain feldspars, nepheline syenite, petalite, bone and plant ashes, whiting, and dolomite.

An assortment of metal oxides or other metal compounds produce particular colors when fired.  These are added in such small amounts to the glaze, that they aren't usually a great hazard.  Luster or metallic glazes are fired in a reduction atmosphere.  These glazes can contain mercury, arsenic, highly toxic solvents such as aromatic and chlorinated hydrocarbons, and oils such as lavender oil.  The common metals are often resinates of gold, platinum, silver, and copper.  Some underglazes and overglazes use mineral spirits as the vehicle instead of water.

Glaze components are weighed, sorted and mixed with water.  These materials are often in fine powdered form, and result in high dust exposures.  Glazes can be dipped, brushed, poured, or sprayed on the ceramic piece.

Hazards

Lead compounds are highly toxic by inhalation or ingestion.  Symptoms of lead poisoning include: damage to the peripheral nervous system, brain, kidney, or gastrointestinal system, as well as anemia, chromosomal damage, birth defects and miscarriages.

Lead-glazed foodware can leach lead if not fired properly, or if the glaze composition is not correctly adjusted.  For example, the addition of copper to lead frits renders a higher solubility of lead in the final fired ware.  Acidic drinks and foods such as tomato juice, citric juices, sodas, tea, or coffee, can increase this hazard.

A glaze label marked "lead-safe" means that the finished ware, if fired properly, will not release lead into food or drink.  The actual glaze is still hazardous to handle and fire and may contain lead.  Adequate control over firing conditions is very difficult in the craft studio.

Other fluxes such as barium and lithium are also highly toxic by inhalation, but less so than lead.

Certain colorant compounds of particular metals are known or probable human carcinogens, including: arsenic, beryllium, cadmium, chromium (VI), nickel, and uranium.

Antimony, barium, cobalt, lead, lithium, manganese, and vanadium colorant compounds are highly toxic by inhalation.

Antimony, arsenic, chromium, vanadium, and nickel compounds are moderately toxic by skin contact.

Free silica occur in many of the clays, plant ash, flint, quartz feldspars, talcs, etc. used in glazes.  See the discussion above for the hazards of silica and the disease silicosis.  Weighing and mixing glazes can result in the inhalation of these toxic materials.

Soda ash, potassium carbonate, alkaline feldspars, and fluorspar used in glazes are skin irritants.

Spray application of glazes is very hazardous because of the potential inhalation of glaze mists.

Dipping, pouring, and brushing certain glazes may cause skin irritation and accidental ingestion due to careless personal hygiene habits.

Glazes containing solvents are both flammable and hazardous.

Precautions

Use lead-free glazes.  If the glaze does not state "lead-free" or "leadless" on the label, assume it contains lead until proven otherwise.

Lead glazes should only be used on non-foodware items.  Design lead-glazed pieces so that they won't be used for food or drink.  Lead-glazed pottery should be labeled as lead-containing.

If possible, don't use colorants that are known human carcinogens and avoid probable human carcinogens.  There is no known safe level of exposure to carcinogens.

Consider wearing a respiratory when weighing and mixing powdered.  Wet glazes are not an inhalation hazard.  Good housekeeping procedures and cleanup of spills reduce the risk of inhalation or ingestion of toxic dusts.  Wet mop spilled powders.

Gloves should be worn while handling wet or dry glazes.

Good dilution ventilation or local exhaust ventilation should be available when applying solvent-containing glazes.

Basic personal hygiene rules should be followed including restricting eating, drinking, or smoking in the studio, and wearing personal protective equipment such as gloves, and separate work clothes or coveralls.  Wash hands after work. Leftover glazes and glaze scrapings can be homogenized, combined, tested, and used as a glaze. 

What do you think about the danger of using glaze? Can you think of any other types of danger that may have not been mentioned.

Please listen to the Video for the needed information on worksheet.