In the world of pottery, the terms flocculation and deflocculation are crucial for controlling the properties of clay slips and glazes. These two opposing processes manipulate the way clay particles interact with each other in a water-based suspension, ultimately affecting the consistency and workability of the mixture.

Here's a breakdown of each concept:

Flocculation: Bringing Particles Together

Imagine tiny clay particles suspended in water. In a flocculated state, these particles are attracted to each other, forming small clumps or "flocs." This attraction is due to the natural electrical charges on the surfaces and edges of the clay platelets.

Key characteristics of a flocculated slip or glaze include:

  • Thicker consistency: The clumping of particles increases the viscosity, making the mixture thicker and less runny. It will have a more gel-like or creamy texture.

  • Improved suspension: The interconnected structure of the flocs helps to keep all the particles, including heavier ones, suspended in the mixture, preventing them from settling at the bottom.

  • Better for brushing and dipping: A flocculated glaze adheres well to bisque ware in a thicker, more even coat without dripping or running. This is ideal for achieving good coverage.

  • Thixotropic quality: This means the mixture becomes more fluid when agitated (stirred or shaken) and thickens again when at rest.

Common materials used to induce flocculation, known as flocculants, are acidic substances. A popular and readily available flocculant for potters is Epsom salts (magnesium sulfate). A small amount dissolved in water and added to a slip or glaze can significantly increase its viscosity. Vinegar can also be used.

Deflocculation: Pushing Particles Apart

Deflocculation is the opposite of flocculation. In this process, the clay particles are made to repel each other. This is achieved by adding a deflocculant, which is typically an alkaline substance.

Key characteristics of a deflocculated slip or glaze include:

  • Thinner consistency: Because the particles repel each other, they can move past one another more easily, resulting in a more fluid and watery mixture without adding more water.

  • Lower viscosity: This allows for a very fluid slip with a high solid-to-water ratio.

  • Ideal for slip casting: Deflocculated slip is perfect for pouring into plaster molds. Its fluidity allows it to fill every detail of the mold, and because it has less water, the cast piece will shrink less as it dries.

  • Can cause settling: If a glaze is too deflocculated, the particles will not be able to hold each other in suspension, leading to the heavier materials settling at the bottom of the container, creating a hard, difficult-to-remix layer.

  • Common deflocculants used in pottery include sodium silicate (also known as water glass) and Darvan. It is crucial to add deflocculants in very small, precise amounts, as over-deflocculating a slip can ruin its properties.

In a Nutshell: Flocculation vs. Deflocculation

Feature Flocculation Deflocculation

Particle Interaction Particles attract and clump together Particles repel each other Consistency Thicker, more gel-like Thinner, more watery Common Use Glazes for brushing/dipping Slips for casting Additive Type Flocculant (e.g., Epsom salts)Deflocculant (e.g., sodium silicate)

[research via AI]

Hydrogen Peroxide in the Pottery Studio: A Double-Edged Sword

Hydrogen peroxide is a chemical compound that has found a niche in the pottery studio, primarily as a potent agent for controlling mold and eliminating unpleasant odors in clay, particularly in reclaimed scraps.1 While its effectiveness in this regard is widely acknowledged by ceramic artists, its broader effects on the physical properties of clay are more nuanced and less understood.

The Primary Role: A Studio Sanitizer

The most common application of hydrogen peroxide in ceramics is to combat the growth of mold and bacteria in moist clay. Clay, being a natural material rich in organic matter, provides an ideal breeding ground for microorganisms, leading to unsightly and often foul-smelling mold growth, especially in reclaim buckets.2

When introduced to clay, hydrogen peroxide (H₂O₂) decomposes into water (H₂O) and oxygen (O₂).3 This release of oxygen creates an environment that is hostile to anaerobic bacteria, which are often the culprits behind the noxious "swampy" smell of old clay. The oxidizing properties of hydrogen peroxide also effectively kill mold spores.4 For potters, a small amount of hydrogen peroxide can quickly freshen a batch of reclaim clay, making it more pleasant to work with. Anecdotal evidence from numerous potters confirms that once the fizzing reaction has stopped, the hydrogen peroxide has broken down, leaving no residue that negatively impacts the firing process.

Effects on Clay's Working Properties: A Gray Area

The impact of hydrogen peroxide on the workability and plasticity of clay is a subject of debate and lacks definitive scientific consensus within the ceramics community.

Some potters believe that the presence of a certain amount of biological growth (often referred to as "aging" or "souring") can actually improve the plasticity of clay. The theory is that the byproducts of microbial activity can act as natural plasticizers. By eliminating this microbial life, hydrogen peroxide might indirectly reduce this perceived increase in workability.

From a chemical standpoint, hydrogen peroxide is a strong oxidizer.5 It can react with various components within the clay body. For instance, it is known to oxidize iron-bearing minerals and can be used to treat clays containing pyrite (iron sulfide) to prevent issues like efflorescence on the fired surface. While this is a recognized industrial application, the direct effect of these oxidation reactions on the feel and performance of the clay in a studio setting is not well-documented.

There is some evidence from soil science that hydrogen peroxide can alter the structure of heavy clay soils by breaking down the bonds between clay particles, which could theoretically impact plasticity.6 However, the concentrations and applications in agriculture differ significantly from those in a pottery studio.

Flocculant or Deflocculant?

The role of hydrogen peroxide as a flocculant (a substance that causes particles to clump together) or a deflocculant (a substance that disperses particles) in pottery slips is not a common or well-understood application. Flocculation and deflocculation are critical for controlling the viscosity of casting slips and glazes. Generally, potters use specific electrolytes like sodium silicate and soda ash for deflocculation and Epsom salts for flocculation. While a patent exists for using hydrogen peroxide in the bleaching of kaolin slips at a very low pH, this process is not typical of standard ceramic practices.7 For the average studio potter, hydrogen peroxide is not a reliable or predictable tool for altering slip viscosity.

Safety Precautions

It is crucial to handle hydrogen peroxide with care. The common household concentration of 3% is generally safe for occasional contact with skin, but higher concentrations can cause chemical burns.8 When using hydrogen peroxide, especially in larger quantities or higher concentrations, it is advisable to wear gloves and eye protection.9 It should always be stored in its original light-blocking container to prevent it from degrading into water and oxygen.

In conclusion, hydrogen peroxide serves as a valuable and effective tool for potters to maintain the hygiene and usability of their clay by preventing mold and odor. However, its effects on the nuanced working properties of clay, such as plasticity, are not straightforward and may be perceived differently by individual artists. Its use as a primary modifier of slip rheology is not a standard or recommended practice in the ceramic arts.

I added hydrogen Peroxide to my slip bucket to combat odor and the reaction was pronounced. In addition to making it odor free, it appears that it also has a very strong flocculation effect. I think this will make it more effective as slip for applying to pieces I’m creating. Also, very likely to be better consistency for slip molding.

As I use this slip and see the affect I’ll follow up on this report.