Pole Treatment Protocol

The 6-step process is identical across all designs and is absolutely non-negotiable. Untreated eucalyptus in Portugal fails in 5-10 years — attacked by subterranean termites, wood-boring beetles, and fungal rot long before it wears out structurally. Properly treated timber lasts 30-40 years above ground, and this sequence keeps poles flexible for bending into arch frames.

Eucalyptus Species Selection

For structural poles, select E. globulus (blue gum) or E. grandis (flooded gum). These species have characteristic bending strength of 25+ MPa. Avoid E. camaldulensis for primary rafters — its strength is more variable. Source poles 15-18cm diameter at the butt end for main rafters.

This is not optional

In southern Europe, untreated eucalyptus faces three concurrent threats: subterranean termites (Reticulitermes grassei — widespread in Portugal from the Algarve to the Tejo valley), wood-boring beetles (house longhorn and furniture beetles that lay eggs in untreated sapwood), and fungal decay (brown and white rot fungi that thrive in the wet winters). Skip treatment and you’ll see the first damage within 2-3 years. Follow this protocol and your timber will last 30-40 years.

1. Debark Within 48 Hours

   Remove bark immediately after felling while wood is fresh. Use a drawknife or scraper. Bark traps moisture and prevents preservative penetration.

2. Char Bottom 50cm (Deep Char Method)

   Torch the lower 50cm of each pole until the char layer reaches 8-10mm depth. You should see alligator-scale cracking in the char layer. After charring, brush off loose char with a stiff wire brush.

   Why Char When Poles Sit on Gabion?

   All BioDome designs place poles on gabion foundations with HDPE wrapping — no direct soil contact. However, 8-10mm charring remains valuable as a redundancy layer:

   • HDPE wrapping has a 15-25 year service life; char protection is indefinite
   • If HDPE gets damaged during construction or maintenance, char layer provides backup
   • Charred surfaces don’t absorb splash moisture from rain hitting the gabion
   • Conservative design: protects against installation errors, unforeseen settling, or drainage failures

   The char depth isn’t because poles touch ground — they don’t. It’s because a single moisture barrier (HDPE alone) isn’t sufficient for a 30-40 year structure. Test depth by scraping with a knife.

   

   Target 8-10mm char depth with visible alligator-scale cracking

3. Borax Soak (72+ Hours)

   Submerge poles completely in borax solution. They float, so weight them down with stones or a weighted board. Minimum 72 hours; longer (up to 2 weeks) is better for deep penetration.

4. Bend into Arch Frame While Wet

   Poles coming out of the borax soak are saturated and at peak flexibility. Assemble the arch frame immediately — clamp, lash, or bolt poles into their curved positions while they bend easily.

   Joint Shrinkage

   Wet eucalyptus shrinks 4-8% radially as it dries. Connections made on wet poles will loosen. Use wedged connections that can be cinched after drying, or plan to re-tighten all lashings once the frame has set.

5. Dry in Place (2-3 Weeks)

   With the arch frame assembled, allow the structure to dry in position. Wood fibers lock into the curved shape as moisture content drops. Protect from rain with tarps if no roof is in place yet. Check joints and re-tighten lashings as the wood shrinks. Use a moisture meter — target below 15% before sealing.

6. Seal with Hot Linseed Oil

   Heat linseed oil to 60-70°C. Brush on all pole surfaces. The oil hardens over 2-3 days and waterproofs the wood while remaining breathable. Reapply every 5-10 years.

Building a Soaking Trough (Modular Sheet Metal)

For batch processing 200+ poles, build a portable above-ground trough from sheet metal sections lined with pond liner. No excavation required — the trough sits on simple supports, can be assembled in an hour, and disassembles flat for transport or storage. Size it to fit the longest poles in your design with clearance:

Dimension	Size	Rationale
Length	Longest pole + 0.5m	Must fit your longest poles with clearance (e.g. 12m for 11.6m arch poles)
Width	60-80cm	Depends on sheet size; fits 4-5 poles side-by-side per layer
Depth	30-40cm	Shallower than a trench — above-ground so no depth lost to ground level
Capacity	Length × 0.7m × 0.35m × 1000	e.g. 12m trough ≈ 2,940L; treats 15-20 poles per batch

Construction Steps

1. Source sheet metal: Corrugated roofing sheets, flat galvanized sheet, old signage, or salvaged panels — anything rigid enough to bend into a U and hold water when lined.
2. Bend into U-channels: Bend each sheet into a broad U shape (~60-80cm wide, ~30-40cm deep). Use a brake or fence post for flat sheet; corrugated sheets fold by hand along the ridges.
3. Set up frame: Place supports (sawhorses, timber trestles, or stacked blocks) along the run at ~1.5m spacing. Level them carefully — water finds every low spot.
4. Line up sections: Lay U-channels end-to-end on the frame, overlapping edges by 10-15cm at each joint.
5. Install pond liner: Drape EPDM (1mm) or heavy polyethylene (200+ micron) along the full length inside the channel run. The liner seals all joints between sections — no need for waterproof connections in the metalwork.
6. Strap it down: Run ratchet straps around the frame and channels every ~1m to hold everything tight and prevent the sides from splaying under water weight.

Borax Solution Recipe

Ingredient	Amount per 1,000L	Cost (approx)
Water	1,000 liters	—
Borax (sodium borate)	12.5kg	€19-25
Boric acid	8.3kg	€12-21

This creates approximately a 2% solution — sufficient for insect and fungal protection. Scale proportionally to your trough volume. Dissolve borax and boric acid in hot water first, then add to trough.

Modular sheet metal trough: U-channel sections on a frame, sealed by a continuous pond liner

Batch Processing Workflow

1. Load batch: Slide 15-20 debarked poles into trough using ramps
2. Submerge: Weight down with stones or a board weighted with concrete blocks
3. Cover: Lay tarp or corrugated sheets over trough to reduce evaporation
4. Soak 72+ hours: Flip poles halfway through if any surfaces exposed
5. Remove and bend: Slide poles out and assemble into arch frames immediately while saturated and flexible
6. Repeat: Solution remains effective for 6-8 batches before needing refresh

Reusing the Solution

The borax solution doesn’t “wear out” — it absorbs into the wood but concentration drops slowly. After 6-8 batches, test by drying a sample: white crystalline residue on the surface indicates adequate concentration. Top up with fresh solution as water evaporates.

After the Build

Disassemble the trough, stack the sheet metal sections flat, and roll up the liner — store for the next build or loan to a neighbor. The dilute borax solution is not hazardous — borax is used in garden soil as a trace mineral (boron). Drain it onto open ground or a garden bed.