Mastering AgyoPosu: The Art and Science of Preparing Hanji

 Chapter 1: “The Foundation of Paper Sizing: Exploring AgyoPosu in Korean Art”

Throughout history, paper has been one of the most popular surfaces for painting in both Eastern and Western art. In particular, East Asian art frequently utilizes paper for its unique absorbency, which lends a soft, fluid quality to ink and pigments. Among the many ways to treat and prepare paper, AgyoPosu, or animal glue sizing, stands out as the most effective technique to control paper’s absorbent properties.

In this series, we’ll explore AgyoPosu’s role in traditional Korean painting, focusing first on understanding why and how we treat paper to adjust its absorbency.

In Korea, hanji—a paper made from the fibers of the mulberry tree—has long been favored for painting and calligraphy. In the West, paper made from wood pulp is more common. Both types of paper are organic and naturally absorbent, as they’re made from plant fibers that attract water. While this absorbency can sometimes pose challenges, it also makes paper an excellent surface for water-based adhesives like animal glue, gum arabic, and tempera.

The absorbency of paper comes from two main features:

Chapter 2: “Absorbency and Structure: Why Paper Behaves Like a Sponge”

Paper has a fascinating structure, almost like a sponge. The reason tissues absorb water so well is due to the countless microscopic pores between their fibers, a phenomenon known as capillary action. Tissues and toilet paper take advantage of this feature to maximize absorbency. Similarly, if you look closely at hanji (traditional Korean paper) under a microscope, you’ll see that its fine mulberry fibers are intricately woven together, much like nonwoven fabric. This structure creates numerous tiny gaps, or pores, that allow water to seep in.

The degree to which paper absorbs moisture or causes colors to “bleed” varies based on the density of these pores. The more pores there are, the greater the bleeding effect. Hanji, with its abundant micropores, creates a noticeable spread of ink or color on the surface.

Interestingly, even if the paper material remains the same, adding a filler like starch or glutinous rice to the fibers can reduce the number of these pores, resulting in less spreading or bleeding. In Korea, there’s also a technique called dochim, which involves repeatedly beating the paper to create a smooth, polished surface. This unique process enhances the paper’s density and gloss. While dochim improves the surface and reduces pores, it doesn’t have the same effect as AgyoPosu in fully preparing the paper for painting, especially for handling water-based pigments. The choice to use fillers or dochim often depends on the desired effect, but it’s important to recognize that each technique influences the absorbency and stability of the paint layer differently.

Understanding how these pores and treatments affect absorbency is essential to achieving desired effects in traditional Korean painting. In the next chapter, we’ll explore why AgyoPosu itself is applied and the unique benefits it brings to painting on hanji.

Chapter 3: “Managing Absorbency: The Necessity of AgyoPosu in Pigment Stability”

In East Asian painting, artists often make the most of paper’s natural absorbency. By adjusting the amount of ink on the brush and the speed of brushstrokes, they can create beautiful effects with dry brush strokes (galpil) and wet brush strokes (yunpil), allowing ink to flow smoothly and naturally. However, this absorbency can become a challenge when working with colored pigments, especially those that contain particles, which need to stay securely on the paper’s surface.

Without proper treatment, the animal glue solution used to fix pigments can either be fully absorbed or simply bleed out, failing to secure the pigments on the paper. For these particles to adhere stably, we need to prevent the glue from soaking into the fibers, ensuring it stays on the surface.

This is where AgyoPosu, or animal glue sizing, becomes essential. AgyoPosu creates a fine barrier on the paper, reducing its absorbency and allowing pigments to rest on top rather than sink in. The result is a stable, vibrant layer of color that remains intact on the surface, even when water-based paints are used. Without this step, the natural absorbency of hanji would draw the glue and pigments into the fibers, making it difficult to achieve the crisp, stable colors that traditional Korean paintings are known for.

In the following chapters, we’ll dive into the roles of animal glue and alum in this process, each playing a crucial part in transforming hanji into a receptive, stable surface for painting.

Chapter 4: “Animal Glue in Depth: The Backbone of Effective Sizing”

Animal glue, made from the collagen in animal or fish proteins, plays a key role in AgyoPosu. Collagen, derived from the Greek words kolla (glue) and gen (origin), has a unique quality: it swells and dissolves in water. This simple yet essential property defines many aspects of Korean color painting, allowing artists to create layers with distinct effects.

For example, the colors in a paint layer often look different when wet compared to when they’re dry, due to changes in the surface structure caused by the glue’s evaporation. The water-soluble quality of animal glue also enables artists to take advantage of pigment particle size, which can affect color perception. It’s no wonder that hydrophilic paper (such as hanji) and water-soluble animal glue-based paints are a natural match.

Applying animal glue sizing to paper serves to fill the tiny pores and create a thin film between the fibers. This controls the paper’s absorbency by providing a slight water resistance. However, since glue is water-soluble, it cannot fully prevent the colors from bleeding on its own. To make the glue resistant to water, a small amount of alum is added to the solution.

One might wonder if simply applying animal glue alone could achieve the effect of AgyoPosu. It’s true that some glue remains within the fibers when applied, filling the pores to a degree. However, when water-based paints are applied later, any remaining moisture would dissolve the glue in the fibers, allowing the paint to bleed.

The role of animal glue in AgyoPosu is therefore twofold: it fills the pores of the paper and forms a thin film that bonds with the fibers. However, to make this barrier fully water-resistant, the addition of alum becomes essential. In the next chapter, we’ll explore alum’s role and how it transforms the glue from water-soluble to water-resistant, enabling artists to achieve long-lasting effects on hanji.

Chapter 5: “Alum’s Essential Role: Transforming Soluble Glue into a Water-Resistant Layer”

Many people think that the alum in AgyoPosu fills in the pores of hanji (traditional Korean paper), acting as a preservative to protect against insects. However, alum’s actual purpose is different—it’s there to change the water-soluble animal glue into a water-resistant layer.

To understand this, let’s revisit the purpose of AgyoPosu. The goal of this sizing technique is to apply a treatment to hydrophilic (water-attracting) paper to prevent water from spreading. However, since animal glue dissolves in water, it can’t provide lasting water resistance on its own. To fix this, alum is added to the glue solution to chemically alter its properties.

When alum is combined with the glue solution and allowed to dry, the previously water-soluble glue no longer dissolves upon contact with water. So, although hanji itself remains hydrophilic, the layer of glue and alum on top makes the surface water-resistant. This allows the pigments to stay on the surface without being absorbed into the fibers.

To dig deeper into alum’s role, alum (or alumen in Latin) is made of potassium aluminum sulfate (KAl(SO₄)₂·12H₂O). The aluminum ions in alum react with the amino acids in the glue’s protein, forming cross-links that prevent the glue from breaking down in water. This reaction creates a water-resistant network within the paper.

As a result, when AgyoPosu is applied, this network forms between the cellulose fibers of hanji, ensuring that the pigments don’t seep into the paper. Instead, the glue solution binds securely between the pigments and between the pigments and the hanji surface, creating a durable, stable paint layer.

Understanding alum’s role is crucial for artists who wish to control absorbency and ensure the longevity of their work. In the next chapter, we’ll discuss the ideal ratio of glue to alum needed to achieve this effect without risking long-term degradation of the paper.

Chapter 6: “Balancing Glue and Alum: Achieving the Ideal Ratio for Lasting Effects”

Since alum’s role in AgyoPosu is to react with animal glue, the amount of alum needed depends directly on the amount of glue used. So, what is the ideal ratio of alum to glue to achieve the best sizing effect?

AgyoPosu works by adding just enough glue and alum mixture to create a water-resistant surface on the paper. Let’s say we start with 10 grams of glue. If there’s not enough alum, the paper will still bleed or absorb moisture. By gradually increasing the alum, we eventually reach the point where the desired AgyoPosu effect is achieved. However, once the glue has reacted with a sufficient amount of alum, adding more won’t increase the water resistance; instead, too much alum can actually harm the paper over time.

Excess alum can cause the organic fibers in hanji to oxidize, as its sulfuric acid content reduces the paper’s moisture over time, causing a gradual pH drop (to around 3–4). This results in discoloration and reduces the durability of the paper. Therefore, knowing the correct ratio of glue to alum not only ensures effective sizing but also helps preserve the artwork for years to come.

Based on the proportion of amino acids in glue that react with alum, about 6% alum relative to the total glue mass is sufficient. For example, if you’re using 10 grams of glue, around 0.6 grams of alum will achieve optimal results.

This fixed ratio remains the same, regardless of the type of paper or its properties. However, artists may want to adjust the concentration of the sizing solution by varying the amount of water. If a lighter coating is desired, more water can be added to dilute the solution; for a stronger coating, reduce the water to increase concentration or apply additional layers of AgyoPosu.

Understanding and controlling this balance of glue, alum, and water enables artists to fine-tune the sizing effect according to their needs. In the next chapter, we’ll discuss specific methods for preparing and applying this balanced AgyoPosu solution effectively.

Chapter 7: “Crafting the Perfect AgyoPosu Solution: Ratios, Conditions, and Application Techniques”

Recommended Ratio for AgyoPosu Solution

Water (350–400g) : Animal Glue (10g) : Alum (0.6g)

Choosing the Right Day to Apply AgyoPosu

Humidity can encourage bacteria growth and weaken the glue’s properties. To ensure quick drying, choose a clear, dry day for applying AgyoPosu. Avoid humid or rainy days, as these can interfere with the sizing process.

Preparing the Glue Solution

To start, prepare 10 grams of animal glue and 300cc of water. Soak the glue in the water for about 3–4 hours to allow it to swell fully. This pre-soaking helps reduce the heating time needed to dissolve the glue, preserving its natural qualities. However, be careful not to soak it too long, as this can lead to bacterial breakdown. Refrigeration can help prevent this if longer swelling is needed.

Heat the swollen glue in a water bath at 60–70°C for 15–20 minutes. The water bath prevents localized overheating and keeps the temperature stable, protecting the collagen chains from breaking down. Use a ceramic or glass container to avoid unwanted reactions with metals like chromium or nickel.

Adding Alum to the Glue Solution

Dissolve 0.6 grams of alum in 100 grams of water to create an alum solution. Once the glue is fully dissolved, the alum should be nearly dissolved as well. For the AgyoPosu solution, about 400 grams of total water is typical, but the amount can be adjusted based on the artist’s intention for a lighter or stronger application. Store-bought glue sticks are flexible but have weaker adhesion, so if using these, consider slightly reducing the water for a higher concentration.

Combine the alum solution with the glue mixture to make approximately 400g of AgyoPosu solution. Adding the alum solution when the glue mixture cools to around 30°C ensures the cross-linking reaction occurs properly, allowing the glue’s protein to bond effectively with the alum.

Applying the AgyoPosu Solution

Use a cloth under the paper to help gauge how evenly the paper absorbs the solution. Apply the AgyoPosu solution uniformly, ensuring the paper is thoroughly soaked. When the paper is fully saturated, it should become slightly transparent, and the cloth below should be evenly visible.

If excess solution pools on the surface, remove it with a dry brush or tissue to maintain an even coating. Avoid brushing multiple times on a single area without saturating it, as uneven application can cause streaking when the paper is dyed later. While repeated AgyoPosu layers can be applied, this method is less efficient than achieving a uniform application in one go.

Applying AgyoPosu correctly is essential to creating a stable, water-resistant layer, allowing pigments to remain vibrant and secure on the surface. This completes the AgyoPosu process, setting the foundation for beautiful, long-lasting works on hanji.

Chapter 8: “Mastering AgyoPosu: A Foundation for Timeless Korean Art”

The art of AgyoPosu, or animal glue sizing, is both an ancient technique and a critical foundation for traditional Korean painting. Through the previous chapters, we’ve explored the structure of paper, the chemistry behind animal glue and alum, and the precise process of creating an ideal surface for painting on hanji. Together, these elements illustrate how AgyoPosu transforms simple paper into a carefully crafted medium that holds pigment beautifully and endures over time.

To recap, AgyoPosu addresses the unique absorbency of hanji by applying a mixture of animal glue and alum. This mixture not only controls the paper’s tendency to absorb water but also ensures that pigments adhere securely to the surface without bleeding or fading. We learned that animal glue, a collagen-based protein, is the backbone of this process, providing flexibility and adhesion. Alum, in turn, acts as a transformative agent, making the glue layer water-resistant so that colors remain vivid and stable on the paper.

The ideal ratio of glue to alum (10g of glue to 0.6g of alum) plays a crucial role in achieving the best AgyoPosu effect. This ratio prevents excessive alum, which could damage the paper over time, and keeps the glue’s properties intact. Likewise, controlling the amount of water allows artists to customize the solution’s concentration, adapting it to their specific needs and desired effect.

Beyond the technical details, AgyoPosu exemplifies the precision, patience, and respect for materials that characterize traditional Korean art. Each step—from choosing the right day to apply the solution to the even, careful application of the sizing—demands attention to detail. The result is a stable, water-resistant layer on hanji that provides a strong foundation for artistic expression.

In mastering AgyoPosu, artists continue a tradition that honors the natural properties of paper while enabling it to carry the weight and beauty of color and form. This knowledge, passed down through generations, is what allows Korean art to endure, blending nature, technique, and artistry in a timeless form.

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