1. How Long Is the Thread From One Silkworm Cocoon?
When you unravel a cocoon made by a silkworm, it becomes one long, unbroken thread. The length of thread from one cocoon varies by source: about 800 to 1,200 m (0.5 to 0.75 miles) in some records, and 1,200 to 1,500 m (0.75 to 0.9 miles) in others [5]. For now, let’s consider it to be about 1 km (0.6 miles). Unlike cotton or wool, which are made by twisting short fibers together, silk is the only natural fiber that comes as a long "filament" [5].
2. Sericin: The Glue-Like Protein That Makes Up a Quarter of Silk Thread
The silk thread from a cocoon is made of two types of protein. It has two long, thin cores called fibroin, and a substance called sericin that glues these two cores together [1]. Sericin makes up about 20 to 30% of the cocoon thread, roughly a quarter, and it dissolves easily in water or alkaline solutions [3]. The process of removing this glue by boiling in hot water with soap is called "degumming," which reveals the two fibroin cores [3]. If sericin remains, the texture feels stiff. When most of the sericin is removed, the silk can gain more luster and flexibility [5]. Part of that "smooth" feeling comes from this washing process.
3. Why Silk's Fibroin Thread Is Shaped Like a Triangle in Cross-Section
If you cut a degummed fibroin thread sideways and look at it under a microscope, the cross-section is an irregular triangle. It is a triangle with rounded corners. When many thin threads are packed closely together, they scatter light finely, according to the Tango Textile Industry Cooperative [2]. English Wikipedia also states that the rounded corners and triangular cross-section are related to silk’s luster [4]. A research paper using an electron microscope (TEM) observed the cross-section of Bombyx mori silk fibers and reported that it is almost triangular with rounded corners [6]. The thickness is about 10 μm (0.01 mm) according to the Tango Textile Industry Cooperative [2]. This is so thin that about 100 threads could fit side-by-side in 1 mm. It is good to remember it is about 1/100th of a millimeter. If the cross-section is a triangle and not a circle, how does light bounce back? We will look at this in the next section.
4. How Light Reflects From Both the Surface and Inside of Silk Fibers
Fibroin fibers are semi-transparent. Because of this, some light bounces off the surface, while some light enters the inside and refracts (bends). These two types of light combine to create a gentle luster, as explained by the Tango Textile Industry Cooperative [2]. One fibroin fiber is said to be a bundle of about 1,000 (or about 2,000 in some sources) even thinner fibrils [2]. Light that enters the inside reflects and refracts repeatedly at various depths. It is said to appear colored due to interference. Some explanations compare this shine to the multi-layered structure of pearls [2]. Many general sources also explain that the triangular cross-section acts like a prism to split light [3].
5. How Synthetic Fibers Imitate Silk's Triangular Cross-Section
According to English Wikipedia, some shiny synthetic fibers imitate the triangular (trilobal) cross-section of silk to create a silk-like appearance [4]. This shows the idea that even man-made threads have their light reflection partly determined by the shape of the cross-section. Silk can sometimes make a "squeaking" sound when rubbed together, called "silk squeak." Japanese Wikipedia introduces this in connection with the triangular cross-section [5], but it is best to view this as a hypothesis at this stage.
6. How to Check Silk's Shine Yourself With a Scarf or Tie
If you have a silk scarf or tie at home, try looking at it by a bright window while changing the angle. When you tilt it, the brightness or color tone might change. If you have nylon or cotton fabric, place them side by side and compare them at the same angle. For official information, the National Agriculture and Food Research Organization and the Tango Textile Industry Cooperative have pages explaining the structure of cocoon thread and silk luster. Details on "silk squeak" and why the triangle cross-section creates luster vary by source. Comparing them might help you see what is confirmed and what is still an explanation.