What's in a Clay? Finding the right minerals for Salt Glaze Pottery | KQED (2024)

FAQs

What are the 3 minerals found in clay? ›

Clay minerals are made up of three members (the illite group), which include illite, glauconite, and muscovite, and display clay-like characteristics, with illite being the most frequent. Illite is generated by alkaline weathering of potassium and aluminum-rich rocks such as muscovite and feldspar.

Remember, a generalized formula for clay is SILICA, ALUMINA, water and various impurities. Thus, the addition of ALUMINA to a glaze in the form of a clay actually introduces two of the three essential parts of a functional glaze: SILICA AND ALUMINA.

Pottery Clay Minerals

Because they can form large deposits, the most common clay minerals used by potters are kaolinite, montmorillonite and bentonite. These minerals are composed of hydrous aluminum silicate.

Clay minerals are composed essentially of silica, alumina or magnesia or both, and water, but iron substitutes for aluminum and magnesium in varying degrees, and appreciable quantities of potassium, sodium, and calcium are frequently present as well.

Clay minerals are most often identified by reflection powder XRD of both oriented and random preparations. Identification is greatly facilitated if the clay fraction is first separated from the rock (bulk sample), to minimize contamination by non-clay minerals.

There are six main types of clay minerals (Swagata, 2003; Uddin, 2018): (i) kaolinite, (ii) smectite group, (iii) illite group, (iv) vermiculite, (v) chlorite and (vi) allophanes.

Glazes need a balance of the 3 main ingredients: Silica, Alumina and Flux. Too much flux causes a glaze to run, and tends to create variable texture on the surface. The texture may vary from shiny, where the glass is balanced, to matte where the excessive flux oxides may form visible, possibly lumpy, crystals.

Salt glazing involves throwing wet salt into the heated kiln while the bisque ware is being fired. If sodium chloride salt is added, hydrogen chloride gas is formed which is highly toxic by inhalation. Hydrogen chloride and water vapor also form corrosive hydrochloric acid which can corrode metal fittings in the area.

Clay minerals mostly form from pre-existing minerals, primarily from rock-forming silicates by transformation, and/or neoformation, where rocks are in contact with water, air, or steam. As Konta (1992) stated 'everything starts in weathering crust'.

Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al₂Si₂O₅(OH)₄), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Oxford Clay (Jurassic) exposed near Weymouth, England.

What is the most common mineral present in the clay? ›

Illite is similar to muscovite and is the most common clay mineral, often composing more than 50 percent of the clay- mineral suite in the deep sea.

Kaolinite, smectite (in particular, montmorillonite/bentonite), illite, and talc are the most widespread clay minerals (Fig. 3). Different clay minerals may form at the same time, but the deposits are usually temperature-zoned.

Clay is natural, fine-grained (<2 or <4 μm), mainly constituted of phyllosilicates, plastic (except flint clays ), and hardens on drying or firing. On the other hand, clay mineral is natural or synthetic, without size criterion, might include nonphyllosilicates, plastic, and hardens on drying or firing.

Clay minerals such as kaolinite, smectite, chlorite, micas are main components of raw materials of clay and formed in presence of water. A large number of clays used to form the different structure which completely depends on their mining source.

Soil minerals are divided into three size classes — clay, silt, and sand (Figure 1); the percentages of particles in these size classes is called soil texture. The mineralogy of soils is diverse.

A clay deposit that is near the site of erosion and alteration. They have more mechanical impurities and fewer chemical and mineralogical impurities. Primary clays have larger particles and less plasticity.

Clay minerals are hydrous aluminium phyllosilicate minerals, composed of aluminium and silicon ions bonded into tiny, thin plates by interconnecting oxygen and hydroxide ions. These plates are tough but flexible, and in moist clay, they adhere to each other.

These properties include: cation exchange capabilities, plastic behaviour when wet, catalytic abilities, swelling behaviour, and low permeability. They give to clay and clay-based minerals higher application in many industries and processes.