Semi-circular and elliptical fanlight geometry represents the pinnacle of Georgian neoclassical mathematical drafting. Derived from classical Roman triumphal arches and refined through Robert Adam's neoclassical pattern books, the fanlight arch is laid out using multi-centered geometric construction (three-centered or five-centered ellipses) to bridge wide entrance apertures with low ceiling heights. Glazing bars radiate from a central hub (the patera or sunburst semicircular boss) outward to the arched perimeter frame. These radial spokes are mathematically indexed at equal angular divisions (typically 12, 16, or 24 radials), intersected by concentric swagged, pointed, or teardrop lead curves (catenary loops) that create the illusion of a glowing, radiant sunrise illuminating the entrance hall.
At a Glance: Key Takeaways
- Mathematical Geometry: True semi-circles for tall entrance porches; five-centered ellipses for broad, low-transom townhouses.
- Trammel of Archimedes Drafting: Historic wooden beam trammel used to scribe continuous elliptical curvature with zero distortion.
- Central Sunburst Boss (Patera): Solid pine or cast lead medallion anchoring the convergence of radial glazing bars.
- Equal Angular Indexing: Dividing the 180° arch into 12, 16, or 24 equal radiating rays with brass dividing calipers.
- Interlaced Catenary Arches: Intersecting circular and Gothic lancet loops forming the famous 'spiderweb' (toile d'araignée) pattern.
- Joinery Rebate Physics: Slender 10–14mm timber astragals rebated 6mm to seat individual glass segments with linseed putty.
1. Drafting the Neoclassical Curve: From True Circle to Five-Centered Ellipse
In early Georgian architecture (circa 1720–1750), doorways were flanked by classical Doric or Ionic columns with full entablatures, leaving ample vertical space for a true semi-circular fanlight ($R = W/2$). However, as townhouse ceiling heights in entrance vestibules became compressed in late 18th-century speculative terraces, architects required wider apertures with lower heights.
To span a 1.5-meter entrance with only a 500mm vertical rise without creating a clumsy flattened segmental arch, architects employed true ellipses. Master carpenters drafted the curve on banker boards using the Trammel of Archimedes or string-and-pin focal methods ($F_1, F_2$). For stone masons and joiners carving curvilinear wood members, the curve was constructed as a five-centered false ellipse, utilizing three distinct radii ($r_1, r_2, r_3$) to establish smooth tangent transitions that prevented optical flat spots along the haunches.
2. Adams Radiance: Angular Divisions, Paterae, and Spiderweb Loops
The visual drama of a fanlight depends entirely on its internal tracery division. Robert Adam revolutionized this by treating the fanlight as a burst of optical light (lux in tenebris):
- The Central Patera (The Sun): Rather than running spokes down to a bare flat transom beam, radials terminate into a semi-circular wooden hub or cast-lead fan patera adorned with sunflower rays or acanthus petals.
- Radial Spoke Indexing: The 180° arch is divided into equal angular intervals. Standard configurations include 12 bays ($15^\circ$ each), 16 bays ($11.25^\circ$), or for grand palaces, 24 bays ($7.5^\circ$).
- The Spiderweb (Toile d'Araignée): Concentric loops intersect the radial spokes. Instead of concentric circles, these loops are shaped as inverted catenary swags, pointed Gothic lancet tips, or teardrops. Where the curves cross the straight spokes, they form delicate diamond panes that refract natural daylight like faceted gemstones.
3. Timber Astragal Joinery versus Composite Metal Framing
Constructing curved glazing bars (astragals) from solid timber required astonishing joinery skill. Curved wood bars had to be cut from wide mahogany or clear yellow pine boards (bois de fil) using curved compass planes and spoke shaves, with the end joints joined into the outer frame using tiny mortise-and-tenon or lap joints pinned with boxwood needles.
Because all-timber bars had to be at least 14mm wide to avoid snapping, late Georgian architects adopted composite metal framing. Slender 3mm flat wrought-iron spokes were let into the timber frame, providing structural rigidity while reducing the visual obstruction of light by 70%. Lead or brass moldings were then pinned to the iron spokes to create delicate, shadowless glazing profiles.
Geometric Layout and Radiating Parameters for Georgian Fanlights
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Arch Geometry | Span-to-Rise Ratio | Radial Division Count | Drafting Construction Method | Structural Glazing Bar Material |
|---|---|---|---|---|
| Semi-Circular | 2 : 1 (Exact semi-circle) | 12 or 16 equal radials | Compass strike from center transom point | Carved mahogany or slow-grown yellow pine |
| Segmental Arch | 3 : 1 to 4 : 1 (Flatter arc) | 10 or 14 radials | Single center point struck deep below door | Timber with iron reinforcement plates |
| True Elliptical | 2.5 : 1 to 3.5 : 1 | 16 or 20 radials | Trammel of Archimedes / focal pin method | Slender wrought-iron flat bar with cast lead cames |
| Five-Centered Ellipse | 2.8 : 1 to 3.2 : 1 | 16 or 24 radials | Five circular arcs tangent at inflection nodes | Composite wrought iron and solder-welded lead |
| Horseshoe / Moorish | 1.8 : 1 (Extended center) | 18 radials | Struck with center elevated above impost line | Cast iron or bronze decorative grille |
Practical Constraints
Every successful decoration project must account for physical and environmental constraints:
Door Slamming Vibration Impact
Historic Subsidence Out-of-Square Openings
Master Drafting and Joinery Protocol for an Elliptical Radial Fanlight
Execute this professional sequence to achieve flawless results without backtracking:
Aperture Scribing and Width/Height Datum
Measure clear opening width ($W$) and springline height ($H$) of door masonry arch with precision laser level; transfer to banker layout board.
Setting Trammel of Archimedes for Ellipse
Set trammel sliders to major semi-axis ($a = W/2$) and minor semi-axis ($b = H$); rotate beam to scribe true elliptical outer line.
Laminating Outer Timber Rim Arch
Bend multiple 4mm clear pine veneers over circular form with resorcinol glue; clamp for 24h to produce stable, warp-free laminated rim.
Laying Out Central Sunburst Patera Hub
Turn semi-circular hardwood patera block on lathe; scribe 16 equal angular radial centers ($11.25^\circ$) across the 180° diameter.
Cutting Radial Spoke Mortises
Cut tiny 3x10mm mortises into central patera and outer perimeter rim to receive the tenoned ends of the radiating spokes.
Bending Concentric Catenary Swag Loops
Form curved lead came or steamed pine loops over wooden form blocks; notch junctions to interlock half-lap with straight radial spokes.
Soldering Metal Joints and Putty Glazing
Solder all came intersections with 60/40 tin-lead solder; bed hand-cut antique cylinder glass panes into Linseed oil glazing putty.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Drafting an elliptical fanlight using a single radius with flattened ends (a simple basket arch)
Dividing the fanlight radials by equal linear distances along the curved arch instead of equal angles
Carving curved timber glazing bars from straight-grained wood across the grain
Project Execution Checklist
Track your physical progress. Checkboxes persist automatically on this device:
Frequently Asked Questions
What is the difference between an Adamesque fanlight and a Queen Anne fanlight?
Queen Anne fanlights (early 18th century) were simple, thick-timbered semi-circles with square glass panes. Adamesque fanlights (late 18th century) are airy, gossamer-thin neoclassical masterpieces featuring wide elliptical curves, radiating sunbursts, urns, and cast-lead swagged loops designed by architect Robert Adam.
Why did architects use multi-centered ellipses instead of true mathematical ellipses?
Before computer CAD, drafting a true ellipse was easy on paper with string, but difficult for stonecutters and joiners who needed precise circular tool paths to carve stone voussoirs and mold timber astragals. A five-centered ellipse approximates a true ellipse using five interlocking compass arcs.
Can an original Georgian fanlight be fitted with modern double glazing?
Generally no. Historic slender glazing bars (10–14mm wide) cannot accommodate thick, heavy modern double-glazed units without destroying their delicate profiles. Instead, conservators install a secondary discreet interior storm panel or use ultra-thin vacuum-insulated micro-glazing (Fineo).
