Coula (Coula edulis) for Epoxy River Tables
Coula (Coula edulis), originating from Africa, is a dense and heavy hardwood traditionally utilized for heavy carpentry, decking, industrial or heavy flooring, poles, sleepers, and sliced veneer. With rich coloration and exceptional strength, it presents a distinctive option for custom epoxy river tables and solid resin-wood furniture. However, its high mass and shrinkage properties require careful attention in the shop. The technical data below is based on research published in the CIRAD Tropix technical sheet for Coula.
| Property | CIRAD Tropix value | What it means / US units |
|---|---|---|
| Botanical name(s) | Coula edulis | |
| Colour / texture / grain | Red brown; texture fine; grain straight or interlocked; interlocked grain slight | |
| Specific gravity (12% MC) | 1.01 | about 63.1 lb/ft³; a 2-inch-thick slab weighs about 10.5 lb per sq ft |
| Monnin hardness (12% MC) | 7.5 | French hardness scale; higher = harder (not a Janka value) |
| Total tangential shrinkage | 8.5 % | green to oven-dry |
| Total radial shrinkage | 4.5 % | green to oven-dry |
| Ratio St/Sr | 1.9 | closer to 1 = less cupping tendency |
| Coefficient of volumetric shrinkage | 0.63 % per % MC | volume change per 1% change in moisture content |
| Fibre saturation point | 23 % | |
| Thermal conductivity | 0.32 W/(m·K) | |
| Crushing strength (12% MC) | 78 MPa | about 11,310 psi |
| Static bending strength (12% MC) | 142 MPa | about 20,600 psi |
| Modulus of elasticity (12% MC) | 19,490 MPa | about 2,827,000 psi |
| Decay (fungi) resistance | Durability class 1 – very durable | |
| Termite resistance | Class D – durable | |
| Dry-wood borer resistance | Class D – durable (sapwood demarcated, risk limited | |
| Heartwood treatability | Class 3 – poorly permeable | |
| Use class from natural durability | Class 4 – in ground or fresh water contact | |
| Drying | rate slow; distortion: high risk; checking: high risk; collapse: no known specific risk; casehardening: no known specific risk | |
| Suggested kiln program temperatures | 45–65 °C | about 113–149 °F |
| Machining | blunting fairly high; saw teeth: stellite-tipped; cutting tools: tungsten carbide | |
| Nailing and screwing | Good but pre-boring necessary | |
| CITES status | This species is not listed in the CITES Appendices (Washington Convention 2025). |
Values from the CIRAD Tropix technical sheet (average values for mature wood, 12% moisture content where noted). US-unit conversions are calculated by BigEpoxy (1 MPa = 145.04 psi; density = specific gravity × 62.43 lb/ft³).
Appearance, Grain, and Slab Potential
Coula displays a heartwood color described as red brown to purplish brown, frequently featuring dark brown veins. The sapwood is clearly demarcated from the heartwood, measuring from 3 to 4 cm in thickness. The wood texture is fine, with grain that runs straight or interlocked (noted as slight), though it can sometimes be wavy. Flat sawn surfaces showcase attractive grain variation.
For builders seeking live-edge slabs, CIRAD logs report diameters from 60 to 80 cm. This provides moderate-width slabs suitable for bookmatched dining tables, side-by-side river layouts, or deep coffee table tops, keeping in mind that 3 to 4 cm on each edge consists of demarcated sapwood.
Weight, Hardness, and Mechanical Strength
Coula is an exceptionally heavy timber with a specific gravity of 1.01 at 12% moisture content. In workshop terms, that equals about 63.1 lb/ft³, meaning a 2-inch-thick tabletop slab weighs approximately 10.5 lb per square foot before pouring epoxy. This substantial weight requires robust table bases and hardware.
On the French Monnin hardness scale—where higher numbers indicate greater hardness—Coula measures 7.5 (this is not a Janka value and should never be read as one). Its mechanical numbers match its density: static bending strength is 142 MPa (about 20,600 psi), crushing strength is 78 MPa (about 11,310 psi), and the modulus of elasticity reaches 19,490 MPa (about 2,827,000 psi). It provides exceptional stiffness and crush resistance under heavy loads.
Shrinkage, Stability, and Epoxy Interface Behavior
Working dense tropical species alongside cured epoxy requires understanding wood movement. Coula has a fibre saturation point of 23%. Its total tangential shrinkage is 8.5%, while total radial shrinkage is 4.5%. This yields a tangential-to-radial shrinkage ratio of 1.9, alongside a coefficient of volumetric shrinkage of 0.63% per % MC change.
Because total tangential shrinkage is relatively high (8.5%), any moisture swings after encapsulation can place severe tension on the epoxy-wood bondline. If the wood dries further after casting, it will pull inward across its width, risking separation or shear at the river interface. Slabs must be fully acclimated to the expected indoor equilibrium moisture content before any epoxy work begins.
Drying Risks, Tooling, and Assembly
Coula has a slow drying rate. The CIRAD Tropix data sheet identifies a high risk of distortion and a high risk of checking during seasoning. Conversely, there is no known specific risk of collapse or casehardening. Standard industrial kiln schedules for this timber operate between 45–65 °C (about 113–149 °F).
In the woodshop, Coula demands power. Its blunting effect is fairly high. CIRAD recommends stellite-tipped sawteeth and tungsten carbide cutting tools to handle its dense grain. For joining, CIRAD notes that nailing and screwing hold well, but pre-boring is strictly necessary to prevent splitting. For gluing, the data notes that because of its very high specific gravity, gluing must be especially performed in compliance with the code of practice.
Natural Durability and CITES Status
Coula possesses outstanding biological resistance. Heartwood decay resistance against fungi is Durability class 1 (very durable). It holds Class D (durable) resistance against dry-wood borers and Class D against termites. Heartwood treatability is Class 3 (poorly permeable), and its natural durability places it in Class 4 (suited for in-ground or fresh-water contact). This species is not listed in the CITES Appendices (Washington Convention 2025).
Practical Pre-Pour Checklist for Coula Slabs
- Confirm log dimensions and edge plan: Account for the 60 to 80 cm diameter and decide whether to retain or strip the 3 to 4 cm sapwood layer.
- Dry completely: Given the slow drying rate and high risk of checking and distortion, verify uniform core moisture content across the slab before planning the pour.
- Sharpen cutting gear: Equip saws with stellite-tipped teeth and route with sharp tungsten carbide cutters to counteract fairly high blunting.
- Flatten with rigid machinery: Use low feed rates and stable tooling, recognizing that the wood requires substantial power to cut clean edges.
- Pre-bore all fasteners: Drill pilot holes for any leveling jigs, form cleats, or base fasteners to avoid splitting this dense wood.
- Prepare the bonding interface: Follow the resin maker’s data sheet for surface preparation, keeping in mind CIRAD guidelines for very high specific gravity species.
- Support the finished top: Plan sturdy leg frames capable of handling a tabletop core density of about 63.1 lb/ft³.
Sources
Published October 31, 2026. Sources are linked in the text.
Header image: original graphic by BigEpoxy.com.
Staff note: CIRAD Tropix records the thermal conductivity of Coula at 0.32 W/(m·K), reflecting its dense physical structure which transfers heat faster than lighter domestic table hardwoods. (Source: CIRAD Tropix 7 technical sheet: Coula.)