When the bottom part of a window frame starts to rot in an old New England house, many contractors take the quickest route: they patch the cavity with epoxy resin or an acrylic insert. On the surface, everything looks neat, and the job can be finished in a few hours. But after 3 years, the problem often returns: the new material ages differently than the old wood, so the repair begins to crack, come loose, or let in moisture again.
Master craftsman Vitalii Klykailo, who restores historic windows in Massachusetts and New Hampshire, proposed a different approach to repairing such damage. He turned to materials that have been used in wooden structures for centuries and combined them with precise measurements and a controlled gluing technique.
In January 2026, the written description of the technology, called “Concealed Structural Prosthesis of Historical Timber,” was registered with the U.S. Copyright Office as a literary work. It is based on sturgeon fish glue, carefully selected old wood for the inserts, and a joint design that the craftsman calls a “concealed lock.”
The Weakness of Synthetic Materials
In his early years, Vitalii worked with epoxy resins, professional-grade PVA, and polyurethane composites - materials that were common on the market at the time. Everything changed after he made several return visits to sites that had been repaired with these adhesives two decades earlier.
“An epoxy shield creates a glass-like, rigid zone inside the frame. But old wood absorbs and releases moisture, so it expands and contracts with changes in humidity and temperature. The joint stays in place, the wood moves, and it tears itself apart along the edge of the bond,” explains the craftsman.
The polymer film creates another problem: it blocks the tiny pores in the wood through which it releases excess moisture. As a result, water remains trapped inside, and the deterioration process only accelerates.
How a Craftsman Discovered Isinglass
Vitalii began searching for answers in traditional craft practices. He studied how 18th and 19th-century craftsmen joined wood without modern synthetic materials, and gradually came across animal glues. Eventually, his attention was drawn to sturgeon isinglass - a glue made from fish swim bladders, well known to restorers of violins and antique furniture.
“Stradivari violins haven’t fallen apart over three centuries and still produce their original tone precisely because they were assembled using animal glue,” says Vitalii. It took some trial and error to perfect the recipe. At first, the glue was overheated, causing it to lose its strength. There were also instances when the solution proved too thin: the dry wood absorbed it quickly, and the insert eventually came loose. Through this trial and error, Klykailo gradually arrived at a precise workflow. Once the method was perfected, he tested it under load in a hydraulic press, applying shear force to a glued sample. When it finally gave way, the crack ran not along the glue line but through the body of the historic wood itself - the bond had held better than the century-old timber around it.
A Restored Church Window in Peabody
One of the clearest demonstrations of the method is the Community Covenant Church in Peabody, Massachusetts, a 1960 building whose tall windows had suffered for years from condensation running down the glass and freezing along the lower zones. At the corner joints and the bottom rail, the wood had rotted through by roughly 40 to 45 percent in the most critical area - the junction of the lower rail and the vertical stiles. Under layers of old paint
the timber still looked solid, but inside it had turned to dust, the corner joint had lost its rigidity, and the frame had sagged under its own weight, threatening the large original pane. On this single window Vitalii made three local structural inserts: one main prosthesis replacing the load-bearing core of the lower rail at the tenon joint, and two smaller “capsule” inserts in the vertical stiles where the fibers had begun to delaminate. The donor wood was matched by density and radial cut so that the direction of the growth rings and the slope of the grain aligned precisely with the original, then bonded with the heated sturgeon glue. After full stabilization and a protective finish coat, the seam became invisible even up close: the boundary between the church’s sixty-year-old timber and the new insert cannot be seen, the surface shows no sagging or cracks. The repair restored the window’s structural strength and airtightness while avoiding a full replacement, preserving the historic fabric of the community’s building.
Stages of Restoration
The restorer works according to a different principle: instead of using a synthetic patch, he replaces the damaged section of the frame with a piece of wood. The insert is selected to closely match the original wood in species, density, and behavior under changing humidity. The work then proceeds in three stages.
In the first stage, Vitalii selects the material for the insert. He takes wood from old stockpiles or from dismantled buildings of the same era. Usually, this is aged oak or larch, which are similar to the original frame in terms of density and behavior in a humid environment.
The second stage involves preparing the adhesive. Isinglass is heated only in a water bath, and the temperature is carefully monitored to ensure it does not rise above 60°C. If the adhesive is overheated, it loses the necessary strength and no longer functions as intended.
The third stage involves the joint itself. The craftsman cuts out the damaged area in a complex shape so that the insert fits into the frame as an integral part of the structure. He creates a complex groove into which the wooden insert fits snugly and is held in place by the shape of the joint itself. The joint is reinforced with wooden dowels pre-boiled in oil and made from the same donor wood species.
“When oil is boiling, it drives air and moisture out of the pores and then fills them. Such a dowel will no longer absorb water, swell, or rot, even if the window is caught in a downpour. And it fits tightly into the hole, as if greased, without splitting the fragile old wood around it,” says Vitalii.
Work Duration and Results
In older window units, the lower part of the frame - where water constantly runs down - usually deteriorates. Most often, only certain sections are damaged, so there’s no need to replace the entire structure. The restorer replaces only the wood that has lost its strength, leaving most of the original frame in place. In the units Vitalii works on in New England, decay in this lower zone typically reaches 20 to 40 percent of the wood, while 70 to 85 percent of the original frame - the upper sash, side posts, and the core of the rails - can be saved.
One such repair takes about 1.5 to 2 days. During this time, the craftsman fits the insert, glues it in place with hot glue, secures it with clamps, and allows the joint to set. Afterward, the unit must sit for a few more days before painting to allow excess moisture to escape from the wood.
Whereas a synthetic patch leaves a visible seam one to three millimeters thick and lasts three to five years, a fish-glue prosthesis becomes invisible and lasts for decades. Most importantly, such an insert can be removed if necessary without damaging the historic wood fibers - something a synthetic seam does not allow.
Application in International Practice
The decay of historic wood poses a serious threat to old buildings in many countries, so the demand for proper restoration of such monuments extends far beyond the United States. International restoration principles - primarily the Venice Charter and the guidelines of the International Council on Monuments and Sites (ICOMOS) - require that any intervention on a historic structure be reversible and minimal. In practice, however, epoxy resins and metal fasteners remain the market default, and instead of preserving the historic frame, they gradually damage the very material they are meant to protect.
Klykailo describes the process as a repeatable protocol that records the adhesive temperature, solution density, condition of the wood before gluing, and curing time. This is crucial in restoration, where the method can be transferred to and reproduced by other craftsmen, rather than depending solely on one person’s experience.
Over the years - from his earliest complex projects in Ukraine, including work on unique architectural monuments such as Cossack-era doors, to his current practice in New England under the company Good Window Repair - Vitalii has restored more than 150 individual window and door units by this full method, across more than 25 separate historic
buildings and private estates. His oldest inserts, made with the complete molecular-splicing method in 2021 to 2022, are now four to five years old and holding without a single problem, having passed full cycles of summer heat and winter frost without any delamination or microcracking. Most of his clients in New England are owners of 18th- and 19th-century historic residences who want to keep their original wavy glass and old timber while making the windows warm and functional, and requests increasingly come from neighboring New England states.
New England alone contains thousands of timber-framed houses, churches, and civic buildings from the 17th through 19th centuries, many of which face the same decay in their most exposed wooden elements. With such structures, it is important not simply to cover up the damage, but to preserve as much of the original wood as possible. That is why the craftsman plans to patent the technology and train other craftsmen in its use, so that hidden wood prosthetics can become a well-understood and reproducible method of restoration for the American historic building stock.
Vitalii Klykailo does not leave a site until the window closes properly and no longer lets in the cold. It is precisely this attention to every detail that distinguishes a restoration that will last for generations from a hastily done repair.
