Fish Glue

Part of Adhesives

Making adhesive from fish bones and swim bladders.

Why This Matters

Fish glue is one of the most accessible adhesives for any coastal or riverside community. Every fish catch generates waste — heads, spines, fins, tails, and swim bladders — that most people discard. These scraps contain collagen, the same structural protein that makes hide glue work, and converting them into adhesive requires nothing more than water, heat, and patience.

Compared to hide glue, fish glue has a key advantage: it remains liquid at room temperature far longer, giving you more working time before it gels. This makes it ideal for complex assemblies where you need to position multiple pieces before the adhesive sets. It also penetrates porous materials more deeply, creating stronger bonds on rough or end-grain wood surfaces.

Fish glue production requires no specialized materials and uses parts of the catch that would otherwise attract flies and vermin. A community that fishes regularly can produce its adhesive supply entirely from waste, freeing animal hides for leather production instead of glue-making. In a rebuilding scenario, this kind of resource efficiency matters enormously.

Raw Materials

Not all fish parts produce equally good glue. The quality and yield depend on the species and the specific parts used.

Best Fish Parts for Glue

Material	Collagen Content	Glue Quality	Notes
Swim bladders	Very high	Excellent	Best source; produces isinglass (see dedicated article)
Heads	High	Good	Abundant, easy to collect
Spines and bones	Moderate-high	Good	Need longer processing
Fins and tails	Moderate	Fair	Cartilaginous, dissolve quickly
Skin	High	Very good	See dedicated fish skin glue article
Scales	Low	Poor	Mostly mineral, little collagen
Guts and organs	Very low	Unusable	Cause spoilage, remove entirely

Best Fish Species

Cold-water species generally produce stronger glue than tropical fish because their collagen is more structured. Good sources include:

• Cod — historically the most important glue fish; abundant heads and large swim bladders
• Sturgeon — produces the finest isinglass from swim bladders
• Carp — common freshwater source with good-sized bones
• Catfish — readily available in many river systems
• Salmon and trout — excellent bones and heads
• Any large-bodied fish with substantial bone structure

Small fish like sardines, anchovies, or minnows are not worth processing individually but can be used in bulk.

Preparation of Raw Materials

Proper preparation is critical. Fish waste decomposes rapidly, especially in warm weather, and contaminated materials produce weak, foul-smelling glue.

Collection and Cleaning

1. Collect scraps immediately after filleting. Do not let fish waste sit in the sun. In hot weather, process within 2-4 hours or store in cold water.

2. Separate parts by type. Swim bladders should be processed separately (they produce a superior product). Heads, bones, and fins can be combined.

3. Remove all flesh, blood, and organs. Scrub bones and heads under running water or in a stream. Residual meat and blood cause the glue to spoil faster and produce a darker, weaker product.

4. Remove fat. Fish oil interferes with bonding. Soak cleaned parts in cold water for 2-4 hours, changing the water twice. Fat floats to the surface and can be skimmed off.

Fat Contamination

Fish oil is the primary enemy of good fish glue. Even small amounts of oil prevent the adhesive from properly bonding. Take the time to clean and soak your materials thoroughly. If the soaking water develops an oily sheen, change it and soak again.

Breaking Down Bones

Large bones dissolve slowly. Improve processing time by:

• Crushing or chopping bones into 2-3 cm pieces with a heavy knife or hatchet
• Cracking heads open to expose interior surfaces
• Splitting spines lengthwise where possible
• Removing and discarding scales (they contribute little collagen but add gritty impurities)

The Extraction Process

Equipment Needed

• Double pot setup (inner pot for fish materials, outer pot for water bath)
• Straining cloth
• Collection vessel for finished liquid glue
• Stirring stick
• Molds for drying cakes (if storing)

Step-by-Step Process

1. Load the inner pot with cleaned, chopped fish parts. Add enough water to just cover the material — typically 1 part fish parts to 1.5 parts water by volume.

2. Heat in the double pot to 65-70°C. Fish collagen dissolves at lower temperatures than mammal collagen, so keep the heat gentle. Maintain this temperature.

3. First extraction (3-4 hours). Stir every 20-30 minutes. The water gradually turns cloudy, then opaque, as collagen dissolves. Bones soften and begin to break apart. When bones are soft enough to crush between your fingers, the first extraction is nearly complete.

4. Strain the liquid. Pour through cloth into your collection vessel, pressing gently on the solids to extract as much liquid as possible. Set the liquid aside.

5. Second extraction. Return the strained solids to the inner pot. Add fresh water — about half the volume of the first extraction. Heat again for 2-3 hours. This pulls remaining collagen from the partially dissolved bones.

6. Strain again and combine with the first extraction liquid.

7. Optional third extraction. Yields diminishing returns but can be worthwhile for large batches. The third extract is weaker and should be used for non-structural applications or added to the next batch’s first extraction water.

8. Concentrate the combined liquid. Heat gently in the double pot with the lid off, allowing water to evaporate until the glue reaches a syrupy consistency. This may take 2-6 hours depending on your starting volume and desired concentration.

What to Expect

• Color: Golden to amber for well-prepared material; dark brown for material that was not cleaned thoroughly or was overheated.
• Smell: Mild fishy odor during processing. Strong, offensive smell indicates spoilage in the raw material — the glue is still usable but lower quality.
• Consistency when done: Warm liquid glue should coat a stick and drip in slow, thick ribbons. When a drop is placed on a cold surface, it should gel within 2-3 minutes.

Properties and Uses

Fish glue has distinct characteristics that make it preferable to hide glue for certain applications:

Advantages over hide glue:

• Remains liquid longer at room temperature (sets in 30-60 minutes vs. 5-10 minutes for hide glue)
• More flexible when cured — less brittle, better for joints that experience vibration or slight movement
• Penetrates end grain and porous surfaces more deeply
• Can be applied at lower temperatures
• Raw materials are waste products from food production

Disadvantages:

• Lower ultimate bond strength than good hide glue
• More susceptible to moisture after curing
• Fishy odor can persist in the dried bond
• Shorter shelf life as liquid (use within 3-5 days or dry into cakes)

Best Applications

• Bookbinding and paper work — flexibility prevents cracking along spines
• Veneer and inlay — long open time allows careful positioning
• Musical instruments — flexibility and acoustic properties valued historically
• Light woodworking — boxes, frames, decorative items
• Sealing and coating — mixed with pigments for paint binder
• Textile stiffening — sizing fabric or stiffening rope

Applications to Avoid

• Heavy structural joints under load — use hide glue instead
• Outdoor or wet-environment bonds — fish glue has poor moisture resistance
• Gap-filling — fish glue shrinks significantly and does not fill voids well

Improving Quality

Clarifying

If your glue is cloudy or contains fine particles after straining:

1. Let the liquid settle undisturbed for several hours.
2. Carefully decant the clear upper portion, leaving sediment behind.
3. For critical applications, filter through multiple layers of fine cloth.

Strengthening

To increase bond strength for structural applications:

• Add a small amount of alum (potassium aluminum sulfate) — about 1% by weight — while the glue is hot. Alum cross-links the protein chains.
• Mix fish glue with hide glue at a 1:1 ratio. The blend has better working properties than pure hide glue and better strength than pure fish glue.

Extending Shelf Life of Liquid Glue

Fresh liquid fish glue spoils within days at room temperature. To extend its usable life:

• Add a pinch of salt per cup of glue — salt inhibits bacterial growth
• Store in a cool location (cellar, springhouse, buried pot)
• Add a few drops of vinegar to lower pH and retard spoilage
• Best solution: dry into cakes for indefinite storage and reconstitute as needed

Scaling Production

For a community producing fish glue regularly:

• Designate collection bins at the fish-cleaning station. Separate swim bladders from bones and heads.
• Process weekly or biweekly depending on catch volume. Accumulate scraps in cold water (changed daily) until you have enough for a worthwhile batch.
• Standardize concentration by the drip test — consistent syrup thickness from batch to batch ensures predictable working properties.
• Dry surplus into cakes during productive fishing seasons for use during winter or low-catch periods.
• Save fish oil skimmed during preparation — it has separate uses as lamp fuel, wood preservative, and leather conditioner.

A community catching 20-30 medium-sized fish per week generates enough waste to produce all its light-duty adhesive needs, reserving hide glue only for heavy structural joints.