One of the most aesthetically damaging failures in embedding projects is when the epoxy “pulls away” from the edges of a cured or embedded object, leaving a visible gap or void. This defeats the purpose of encapsulation and often signals a significant difference in how the epoxy and the embedded item behave during the cure.

This issue is primarily caused by differential shrinkage and poor wetting, not contamination.

The Root Causes of Epoxy Gapping

Epoxy, like most plastics, undergoes a small amount of shrinkage as it converts from a liquid to a solid (polymerization). When two materials with different shrinkage rates are bonded together, the more rigid material (the epoxy) pulls away from the less compatible or more mobile object.

1. Differential Curing Shrinkage (The Stress)

• Exaggerated Cure Shrinkage: Standard laminating or coating epoxies often have a cure shrinkage rate of around 2% to 3%. When used for deep, full encapsulation, this small shrinkage is magnified across the volume, creating significant tension that pulls the epoxy inward.
• Rigid Object Resistance: When the epoxy shrinks, it creates a compressive or tensile force on the embedded object. If the object (e.g., a piece of glass, a polished rock, or metal) is completely rigid and non-porous, the epoxy can fail to maintain a tight bond to the side walls, resulting in a visible gap, especially at the top surface.

2. Lack of “Wetting” or Poor Adhesion

• Slick Surfaces: If the embedded object has a very smooth, slick, or polished surface (like glass, ceramics, or highly finished metals), the liquid epoxy may not properly “wet out” or cling to the surface tension of the object during the pour.
• Invisible Residue: Even a microscopic layer of wax, fingerprint oil, or a quick-release spray used to treat the embedded object (to protect it during placement) can prevent the epoxy from forming the initial tight contact needed to resist the inevitable cure shrinkage.

3. Outgassing from the Object Itself

• Porous Objects: If the embedded object is porous (like unsealed wood, bone, or natural stone), the heat from the epoxy cure can cause the object to outgas air or moisture. This outgassing forces a tiny layer of air between the object and the liquid epoxy, creating a visible void line around the object’s perimeter that cannot be filled once the epoxy gels.

Genuine Solutions for Seamless Encapsulation

Eliminating gaps requires using low-shrinkage resins, proper preparation of the embedded object, and a specific pouring technique.

1. Choose a Low-Shrinkage Resin (The Chemistry Fix)

• Use Casting Resins: For embedding and encapsulation, always use a specialty Deep Pour or Casting Resin. These materials are specifically formulated to have a much lower cure shrinkage rate (often under 1%) than standard coating epoxies. Less shrinkage means less pulling force on the embedded object.
• Pour in Stages (Even with Casting Resin): Even with low-shrinkage resins, pouring in thinner layers (within the maximum recommended depth) helps dissipate heat, slows the cure, and minimizes total shrinkage stress in any one batch.

2. Prepare the Embedded Object (The Surface Fix)

• Roughen Slick Surfaces: For non-porous, slick objects like glass or polished stone, etch the surface where it will meet the epoxy. Use coarse sandpaper (e.g., 80-120 grit) or a diamond wheel to lightly scuff the contact areas. This creates a mechanical “tooth” that the epoxy can key into, drastically improving the adhesion against shrinkage forces.
• Clean Aggressively: Ensure the object is meticulously clean. Wipe it down with acetone or IPA to remove all fingerprint oils, polish residue, and dust. Crucially, do not touch the object with bare hands after the final cleaning.
• Seal Porous Objects: Any object suspected of containing air or moisture (wood, concrete, porous stone) must be coated with a thin layer of epoxy seal coat before the final pour. This locks in the air and prevents it from outgassing when the final, thick coat heats up.

3. Application Technique (The Visual Fix)

• Pre-Coat the Object: Before placing the object in the mold, mix a very small, fresh batch of epoxy and use a clean brush to paint a thin coat onto the object, especially around the edges and surfaces that will be visible. This ensures the epoxy wets out every micron of the surface before the object is submerged, eliminating air pockets and gaps.
• Ensure Full Submersion: When placing the object in the final pour, observe closely to ensure no air bubbles are clinging to the surface, especially underneath. Use a small, clean utensil to gently agitate around the edges to release any trapped air.