Mulberry silk is made by harvesting the cocoons of Bombyx mori silkworms, softening them in hot water to loosen the natural protein glue holding the fibers together, then unwinding each cocoon into a single continuous filament that can stretch hundreds of meters long. These filaments are twisted together into raw silk thread, cleaned, and woven into fabric. The entire process, from raising the silkworms to producing finished silk, takes roughly six to eight weeks and has remained fundamentally the same for thousands of years.
Raising the Silkworms
The process starts with silkworm eggs, which are kept in a controlled environment at around 77°F until they hatch into tiny larvae. These larvae feed exclusively on fresh mulberry leaves, which is what gives this silk its name and its reputation. The mulberry diet produces a finer, more uniform fiber than what wild silkworms create eating other plants.
Over the course of about 25 to 30 days, the larvae molt four times and grow to roughly 70 times their original size. When they’re ready to pupate, each worm begins spinning a cocoon by extruding liquid silk protein from two glands near its head. The worm moves its head in a figure-eight pattern, layering a single continuous filament around itself. This takes about two to three days, and the finished cocoon is roughly the size of a peanut shell.
The filament is made of two proteins: fibroin, which forms the structural core, and sericin, a gummy coating that acts as a natural glue binding the cocoon together. Getting usable silk thread means separating these two proteins at the right stage.
Sorting and Stifling the Cocoons
Timing matters. If the moth inside is allowed to emerge naturally, it secretes an enzyme that dissolves a hole through the cocoon wall, breaking the continuous filament into short, unusable pieces. To preserve the full-length fiber, producers kill the pupae inside before they can break through. This is typically done with dry heat or steam, a step called stifling. The cocoons are then dried to prevent spoilage during storage.
Before cooking, workers sort the cocoons by hand or machine, removing any that are stained, double-layered, or malformed. Only well-formed cocoons with intact shells move forward to reeling. Defective cocoons get set aside for lower-grade spun silk products.
Cooking the Cocoons
The critical step in silk production is cooking, which softens the sericin glue so the filament can be unwound without snapping. According to the Central Silk Board, this involves a carefully controlled series of water baths at different temperatures.
A cage of cocoons is first immersed in water at 50 to 55°C for about 45 to 60 seconds. The cage then moves to a hotter bath at 90 to 93°C for 90 to 120 seconds, then to a cooler bath at 60 to 65°C for another 45 to 60 seconds. It returns to the hot bath, now at 95 to 96°C, for another 90 to 120 seconds. Finally, cold water is sprinkled over the cocoons for about 10 seconds to halt the process.
This gradual heating and cooling cycle allows water to penetrate evenly through the cocoon shell, softening the sericin uniformly from outside to inside. If the cocoons are heated too aggressively, the outer layers soften while the inner layers stay glued tight, causing the filament to break during reeling. After cooking, the floating cocoons are pressed with a perforated plate to push out excess water and air.
Reeling the Filaments
Each softened cocoon contains a single filament that can be 300 to 900 meters long, but it’s incredibly thin. A single strand is too fine and fragile to use on its own, so the reeler brushes several cocoons at once to find their loose filament ends, then threads them together onto a reel. Typically four to eight filaments are combined and twisted lightly to form one thread of raw silk.
As each cocoon runs out of filament, the reeler seamlessly adds a new one to keep the thread continuous. Skilled reelers can maintain a consistent thread thickness throughout this process. The result is a skein of raw silk that still contains most of its sericin coating, giving it a stiff, slightly yellowish appearance that looks nothing like the glossy fabric in a finished product.
Throwing and Twisting
Raw silk threads are further processed through a step called throwing, where multiple reeled threads are twisted together to create stronger yarn. The direction and tightness of the twist determine what kind of fabric the yarn will become. A loose twist produces soft, flowing fabrics like charmeuse. A tight twist creates crinkled textures like crepe. The twisted yarn is wound onto bobbins, ready for degumming.
Degumming the Silk
Raw silk still carries 20 to 30 percent of its weight in sericin. Degumming removes this residual glue to reveal the soft, lustrous fibroin fiber underneath. The simplest method uses hot water and mild soap, since sericin’s amino acid structure is hydrophilic. Heat and water break the hydrogen bonds holding the sericin together, dissolving it away from the fiber.
For more controlled results, producers sometimes use alkaline solutions or enzyme treatments with proteases that specifically target sericin without damaging the fibroin. After degumming, the silk loses a significant portion of its weight but gains the smooth, luminous quality people associate with finished silk fabric. The threads are then rinsed, dried, and may be bleached or dyed before weaving.
Weaving and Finishing
The prepared silk yarn is woven on looms into fabric, with the weave pattern determining the final texture and drape. A plain weave creates organza or habotai. A satin weave, where threads float over several others before interlocking, produces the classic high-shine surface of charmeuse. A twill weave results in a subtly textured, durable fabric.
After weaving, the fabric may go through additional finishing steps: calendering (pressing between heated rollers for extra smoothness), dyeing if it wasn’t dyed at the yarn stage, or treatments to improve wrinkle resistance. The finished product is then inspected and graded.
How Silk Quality Is Graded
Finished mulberry silk is evaluated on an international grading scale that runs from A (lowest) to 6A (highest). The grading system, developed by the International Silk Association, measures the fundamental character of the fiber: filament length, purity, uniformity, and color. Grade 6A represents the longest, strongest, and most uniform filaments available. These fibers are pearly white and produce smooth, durable, high-luster fabric. Lower grades may have shorter fibers, less uniformity, or slight discoloration, and they typically end up in less expensive products.
Peace Silk: A Different Approach
Traditional silk production kills the silkworm inside the cocoon before reeling. Peace silk, also called Ahimsa silk, takes a different approach by letting the silkworm complete its metamorphosis and emerge as a moth naturally. Cocoons are collected only after the moth has left.
The tradeoff is significant for the final product. When the moth exits, it pierces the cocoon and breaks the continuous filament into short pieces. These shorter fibers can’t be reeled in the traditional way. Instead, they’re spun like cotton or wool, producing a yarn with a softer, matte texture and visible slubs (small irregularities) that give the fabric a more organic appearance. Peace silk costs more due to lower yields and the additional labor involved in spinning shorter fibers, but it appeals to buyers looking for a cruelty-free option.