Factors Contributing to Glue Creep in Woodworking
Glue line telegraphing (glue-creep) occurs on exposed wood joints as a thin, slightly raised bump along adjacent glued surfaces. This study was designed to evaluate what factors may contribute to this taking place for wood to wood applications. Over the last year, in conjunction with The Gorilla Glue Company, we tested a number of scenarios across a variety of seven wood species, seven adhesive types and four finishes. Initially, when the test samples acclimated within a stable environment, no visible glue-line creep appeared. However, longer-term natural seasonal changes in Cincinnati, OH as well as lab-controlled environmental fluctuations of temperature and humidity, produced visible results of “creep” in two conclusive areas:
- Water soluble adhesives tend to be more prone to creep, particularly when exposed to extreme moisture and temperature.
- Glue-ups with boards containing opposing grain direction (flat sawn -to-quarter sawn) produced a stepping effect based on the tangential (opposing) wood movement.
Woodworkers, contractors and consumers in general have identified glue creep as an occurrence that takes place across a wide range of situations. While adhesive types are typically the focus of why it may or may not occur, nothing published has indicated one adhesive’s performance over another. In fact, no less than 5 different manufactured adhesive brands, along with a series of environmental, mechanical and chemical mechanisms are suspected to effect glue creep.
Documented factors contributing to glue creep have touched three primary areas:
- Environmental Factors: Moisture content, temperature/humidity at glue-up, cure time, seasonal movement, assembly location
- Mechanical Factors: Clamping force, wood porosity, method of application, clamp time, species, grain orientation, and surface prep
- Chemical Factors: Type of adhesive, finish type, adhesive/finish combination, brand formulations
Materials and Methods:
All wood samples mositure content was measured below 12% at the time of milling. Materials consisted of Poplar, Walnut, Cherry, Ash, Maple, African Mahogany and Purple Heart. Each sample was evenly cut and milled from the same wood source (a single board/tree). Every surface that was to receive glue was also given a few light passes with a hand plane to ensure an even and flat surface across all pieces and stabilized for 24 hours prior to glue-up.
For both rounds of testing each board was glued up and clamped using three equally placed bar clamps. Every board was given a full 24 hours of cure time Another 24 hours passed following sanding before the first sample of finish was applied. For Phase I, each board was broken out into four total surfaces for finish application, two halves for each side. Finishes included: Minwax Wipe-on Poly (satin finish); Watco Danish Oil; Zinsser Bulls-Eye Shellac; Unfinished surface (control), Phase 2 testing utilized only wipe-on poly.
For phase I and II of testing we initially left all samples in a consistent environment for a period of 6 weeks, which maintains a relatively consistent temperature of 68 degrees year round, and a relative humidity of ~35-45%. Then, both sets of test boards were conditioned at a warm temperature and high humidity (80F/80%RH) for a total of 19 days, inspecting the boards after days 5, 12 and 19.
For phase I, across all six wood samples, none of the seven glue varieties produced noticeable glue creep. Although some of the lighter and more smooth grained woods produced a visible glue line (particulalry cherry and poplar), neither was consistent from one glue type to the next, with exception to the darker-tinted Titebond III.
For phase II the following observations were made visually and by touch focusing on changes occurring to the faces of the boards.
- The board glued with Gorilla Glue Dark Polyurethane (waterproof) was by far the best (smoothest) of all and did not exhibit any creep.
- Board made with Tite Bond Hide Glue exhibited signs of visible glue creep and joint failure.
- The other 5 boards do not seem to consistently show signs of glue creep. While changes can be felt along many of the joints, it is much more likely that what is being felt is differences in board heights due to expansion in varying directions causing some boards to be thicker/thinner than others. This was most common when differing grain directions are joined together (flat to quarter sawn). Even though edges can be felt, it cannot be concluded that this is glue creep.
As a caveat, wood is cantankerous and often unpredictable. In-fact, the Japanese have a great word for wood’s imperfect beauty: “Wabi-Sabi” or an aesthetic sometimes described as one of beauty that is “imperfect, impermanent, and incomplete”. It will do what it wants, despite all efforts to understand it. After studying various grain direction bonding and high temp/humidity exposure, it has yet to be determined what actually causes any one formulation of PVA glue to consistently creep more than another. Based on our research, as well as a wealth of information available to the online woodworker, there are roughly four key principles that will significantly impact the integrity and performance of any adhesive that woodworkers should be kept in mind.
1. Clean and flat surfaces to be bonded are the foundation of a good glue-up. Glue is not a substitute for poor joinery. The strength of just about every joint relies on a good wood-to-wood bond (including clamping strength).
2. Wood is a bundle of straws. Dead or alive it responds to changes in the environment. Glue-up assembly and and strategies always need to reflect this. Some species simply move more than others.
3. Whenever possible, mate boards of similar grain orientation together when the glue line will be exposed (panels, table tops, etc.). Flat or rift sawn versus quarter sawn boards have very different things going on when it comes to natural wood movement. This is less about glue-creep, and more about two pieces of wood whose thicknesses have changed relative to one another. This produces a stepping effect from one board to the next.
4. If what you are gluing up will be exposed to water or dramatic changes in environmental moisture, you may be better off not using a water based glue, particularly Hide glue.