Working with Cumaru in Epoxy River Tables
Cumaru, also known commercially as Tonka*, encompasses tree species in the genus Dipteryx, including Dipteryx alata, Dipteryx micrantha, Dipteryx odorata (synonymous with Coumarouna odorata), and Dipteryx polyphylla. Sourced from Latin America, this tropical hardwood is renowned for extreme physical strength and heavy construction utility. When builders adapt these timbers for epoxy river tables and live-edge furniture, understanding the raw timber data documented in the CIRAD Tropix technical sheet is critical to managing wood movement, tool wear, and resin adhesion.
Appearance, Figure, and Slab Dimensions
Heartwood tones in Cumaru range from yellow brown in Dipteryx odorata to reddish brown in Dipteryx alata, displaying a primary color reference of red brown accented by darker thin veins. The wood presents a medium texture and a marked interlocked grain. Its sapwood is clearly demarcated from the heartwood, measuring from 2 to 3 cm in thickness. Sourced logs typically range from 50 to 90 cm in diameter, offering solid widths for edge slabs, though the species does not float in water.
Density, Weight, and Monnin Hardness
Cumaru is an exceptionally dense hardwood. At 12% moisture content, it carries an average specific gravity of 1.07. On the shop floor, this translates to a density of about 66.8 lb/ft³, meaning a 2-inch-thick tabletop slab weighs approximately 11.1 lb per square foot. Builders must design robust table bases and heavy-duty joinery to support this mass.
Resistance to indentation and surface wear is reflected in its Monnin hardness rating of 13.1 at 12% moisture content. The Monnin scale is a French measurement system where higher numbers indicate greater resistance to indentation; it is not a Janka value and should not be converted to one. This extreme hardness is matched by tremendous mechanical strength: a crushing strength of 103 MPa (about 14,940 psi), static bending strength of 170 MPa (about 24,660 psi), and a modulus of elasticity of 26,610 MPa (about 3,859,000 psi).
Shrinkage and Movement Next to an Epoxy River
Dimensional change between wood and cured epoxy is the leading cause of seam failure. Cumaru exhibits the following shrinkage characteristics:
- Total tangential shrinkage: 7.7%
- Total radial shrinkage: 5.5%
- Ratio (St/Sr): 1.4
- Coefficient of volumetric shrinkage: 0.73% per % moisture content
- Fibre saturation point: 22%
A tangential-to-radial shrinkage ratio of 1.4 indicates a relatively balanced movement pattern across plainsawn and quartersawn faces, presenting a slight risk of distortion. However, with a volumetric shrinkage rate of 0.73% per percent change in moisture content, any ambient humidity shifts will induce notable dimensional stress against a rigid epoxy interface. Thorough acclimation prior to resin casting is mandatory.
| Property | CIRAD Tropix value | What it means / US units |
|---|---|---|
| Botanical name(s) | Dipteryx alata, Dipteryx micrantha, Dipteryx odorata, Coumarouna odorata (synonymous), Dipteryx polyphylla, Dipteryx p.p. | |
| Colour / texture / grain | Red brown; texture medium; grain interlocked; interlocked grain marked | |
| Specific gravity (12% MC) | 1.07 | about 66.8 lb/ft³; a 2-inch-thick slab weighs about 11.1 lb per sq ft |
| Monnin hardness (12% MC) | 13.1 | French hardness scale; higher = harder (not a Janka value) |
| Total tangential shrinkage | 7.7 % | green to oven-dry |
| Total radial shrinkage | 5.5 % | green to oven-dry |
| Ratio St/Sr | 1.4 | closer to 1 = less cupping tendency |
| Coefficient of volumetric shrinkage | 0.73 % per % MC | volume change per 1% change in moisture content |
| Fibre saturation point | 22 % | |
| Thermal conductivity | 0.34 W/(m·K) | |
| Crushing strength (12% MC) | 103 MPa | about 14,940 psi |
| Static bending strength (12% MC) | 170 MPa | about 24,660 psi |
| Modulus of elasticity (12% MC) | 26,610 MPa | about 3,859,000 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 4 – not permeable | |
| Use class from natural durability | Class 4 – in ground or fresh water contact | |
| Drying | rate slow; distortion: slight risk; checking: high risk; collapse: no known specific risk; casehardening: yes | |
| Suggested kiln program temperatures | 44–60 °C | about 111–140 °F |
| Machining | blunting fairly high; saw teeth: stellite-tipped; cutting tools: tungsten carbide | |
| Nailing and screwing | Good but pre-boring necessary | |
| CITES status | The species in the genus Dipteryx are listed in Appendix II of CITES (Washington Convention 2025). The products concerned are logs, sawn wood, veneer, plywood and engineered wood. Notes. * Common commercial name |
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).
Drying Behavior and Casehardening Risks
Drying Cumaru requires extreme patience. CIRAD Tropix notes that the drying rate is slow and must be conducted carefully. While the risk of distortion is slight and there is no known specific risk of collapse, the species suffers from a high risk of checking. In thick slabs commonly used for river tables, there is a distinct risk of casehardening. Suggested kiln programs begin at mild temperatures between 44 and 60 °C (about 111 to 140 °F) to control surface stress and minimize internal cracking.
Machining, Fastening, and Epoxy Adhesion
Sawing and machining Cumaru are difficult tasks due to its combination of marked interlocked grain and high density. The wood exerts a fairly high blunting effect on tooling. Cutters should utilize tungsten carbide tooling, and saws require stellite-tipped teeth to maintain clean edges without burning.
For joinery and mechanical attachment, nailing and screwing properties are good, but pre-boring is strictly necessary to prevent splitting. When it comes to adhesives, CIRAD Tropix highlights that because of its very high specific gravity, gluing must be especially performed in compliance with professional codes of practice. Heartwood treatability falls under Class 4 (not permeable). Before pouring an epoxy river, wood faces must be thoroughly cleaned, correctly profiled, and sealed according to the resin manufacturer’s technical instructions to foster mechanical grip.
Natural Durability and Trade Status
Cumaru heartwood boasts superior biological resistance:
- Fungi resistance: Durability class 1 (very durable)
- Dry-wood borers: Class D (durable, risk limited to sapwood)
- Termites: Class D (durable)
- Natural durability use class: Class 4 (suitable for in-ground or fresh water contact)
Because of these traits, its conventional end-uses include decking, heavy carpentry, hydraulic works in fresh water and seawater, bridges, industrial flooring, ship building decks, cooperage, sleepers, turned goods, and sliced veneer. In trade regulations, species in the genus Dipteryx are listed in Appendix II of CITES (Washington Convention 2025), covering logs, sawn wood, veneer, plywood, and engineered wood.
Is it a good epoxy table wood?
Cumaru can produce an exceptionally striking, nearly indestructible epoxy river table, but it is not a beginner-friendly wood. Its high specific gravity (1.07), extreme hardness (Monnin 13.1), and marked interlocked grain demand carbide tooling and careful machining. Furthermore, thick slabs run a high risk of checking and casehardening if not seasoned with vigilance. If your stock is thoroughly dried, stabilized, and joined following strict gluing standards, Cumaru delivers outstanding durability and rich reddish-brown figure.
Sources
Published October 30, 2026. Sources are linked in the text.
Header image: original graphic by BigEpoxy.com.
Staff note: Freshly sawn Cumaru produces an unpleasant odour when green, so makers should ensure shop spaces are well-ventilated during initial surfacing and dimensioning. (Source: CIRAD Tropix 7 technical sheet: Cumaru / Tonka*.)