Thirty millimetres, not twenty
We cut granite at 30 millimetres, ten more than marble, because a flamed finish needs material to give away. Flaming runs an 1,800-degree flame across the face for a few seconds; the quartz at the surface expands instantly and spalls off to a depth of 1.5 to 2 millimetres, leaving a texture like orange peel. Take two millimetres off one side of a 20-millimetre slab and what remains is too thin to mitre a corner reliably.
Cutting is by waterjet rather than diamond blade. Bangka and Lampung granite carry more than 30 per cent quartz and eat blades quickly; a waterjet cuts with water and garnet abrasive at 4,100 bar and leaves an edge with no micro-cracking. A micro-cracked edge looks perfect on day one and then spalls in year three, precisely along the line your hand rests on.
Wet grip: 0.68 against 0.21
| Granite finish | Wet friction | How it feels | Where we use it |
|---|
| Flamed | 0.68 | coarse, like orange peel | floor and seats |
| Leathered | 0.54 | softly undulating | inner walls and rim |
| Honed | 0.45 | flat matt | outer faces |
| Polished | 0.22 | slippery when wet | never in a wet area |
The floor and the seats are always flamed. The rim and the inner walls are leathered, because flamed texture is too coarse to lean against for long and your back will feel raw after four minutes. Two finishes in one tub adds about Rp 3.4 million because the slabs are processed separately, and of everything on this page it is the money we think is best spent.
Eight hundred and sixty kilograms: work out the base first
A filled 400-litre granite tub weighs 860 kilograms over a footprint of about 1.5 square metres, which is 575 kilograms per square metre. Terrace tiles over a residential ground slab normally cope. A timber deck, paving over fill, or a first-floor slab almost never do. In a garden we pour a 120-millimetre reinforced pad 30 centimetres larger than the tub on every side, over compacted sand and geotextile. That is Rp 3.5 to 6 million and your own builder can do it from our drawing — who does what is set out under installation and cold test.
Footprint load, 400 L filledabout 575 kg/m²
Pad we ask for120 mm reinforced concrete
Waterjet pressure4,100 bar, garnet abrasive
Flamed spall depth1.5-2 mm
Joints2 mm black epoxy, UV stable
Heat load, 400 L covered, Jakarta516 W
Cladding service life20+ years
Granite and brackish water
In Batam, Pekanbaru and parts of Samarinda the well water carries high chloride and dissolved iron. Granite itself is entirely unaffected; what suffers is every metal component near it. So a granite tub for those cities gets a 316 stainless rim, 316 fixings and a titanium heat exchanger in the chiller, plus a 50-micron pre-filter and ion-exchange media on the inlet. Without that, iron deposits leave brown staining in the pores of the stone which then has to be acid-treated to remove — and acid is the one treatment you want to avoid using on a tub at all.
Two-millimetre epoxy joints, never silicone
The joints between slabs are filled with two-part epoxy at two millimetres, not silicone. Silicone is cheap, flexible and easy to apply, and three things happen to it in an ice bath: it shrinks about five per cent as it cures and pulls away from the stone edge, its surface grows black mould within a year in permanently humid conditions, and it does not bond well to sealed porous granite. Epoxy does not shrink, does not support mould, and is strong enough to help hold the slabs. The trade-off is that epoxy is rigid and unforgiving of movement, which is why the core behind granite is light reinforced concrete rather than a steel frame that would expand at a different rate.
We colour the grout from granite dust saved off your own tub's cutting, so the joint line is the colour of your stone rather than a factory grey. Enough dust for future repairs is kept in a numbered container for five years.
When granite is the wrong choiceWeight is the main obstacle. For a roof terrace, a first floor, a balcony or a timber deck, granite is nearly always outside the structure's capacity and we will decline to install without a calculation from your own structural engineer. Granite is also hard on what falls onto it: a glass bottle breaking on the floor will chip the arris of a joint, where in an acrylic tub it would only scratch. Above ground level look at the acrylic ice bath or 316 stainless, at under a quarter of the weight.
Price and choice of stone
Granite tubs from 300 to 700 litres run Rp 72-125 million. Bangka grey and Lampung black sit at the lower end; imported Indian granite with large even grain adds Rp 8 to 16 million. The chiller follows volume exactly as with any other material, so check yours on the chiller calculator or compare every shell side by side on the pricing page. One warning we always give at handover: granite is extremely hard but its arrises are brittle, so keep glass bottles off the surround and use plastic or metal instead.
Does granite absorb body oil or odour?
Water absorption is 0.2 to 0.4 per cent, far below marble. With the impregnating sealer we apply at handover, body oils stay on the surface and rinse off. Resealing is every three years rather than annually.
Can I use chlorine?
Yes, at 1 to 3 ppm. Granite reacts with neither acid nor chlorine. What still needs protecting is the metalwork and the chiller heat exchanger, so tablets must be dissolved first rather than dropped into the tub.
Does the flamed surface hurt?
Not in normal contact, but it feels coarse if you lean on it for minutes. That is exactly why only the floor and seats are flamed and the walls and rim are leathered, which is much kinder to a back.
How does granite behave in direct sun?
The stone neither fades nor cracks under ultraviolet, so the body is safe. What changes is the heat load: full midday sun adds roughly 210 watts per square metre of water surface, so we either move the chiller up a step or fit shade. The economics of that choice are on outdoor cold plunge.
Can it be made with rounded corners?
It can, either with thermally bent slabs or with segmented pieces. Both add 20 to 30 per cent and a week. For a genuinely curved form, cast terrazzo is far more sensible, because free shape is its natural advantage.
