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Artificial Rock Construction

Constructing Artificial Rocks for Bonsai

from an idea by Brian Mills

It is very rare to find just the right natural rock for a tree. I have looked for years for certain types of rock and had almost given up. The idea that Brian gave me, on the IBC, allows any size and shape of rock to be created. I have paraphrased some of his information and added my own findings, after creating my own first artificial rocks.

It pays to look carefully at natural examples of root over rock and trees planted on rocks to help decide on the style you would like to achieve. Then you can try to imitate natural geological forms. The really convenient thing about this method is that the rock is very, very light in weight. The only disadvantage is that great care will be needed to secure it in the pot to stop it blowing over, even before the trees are added.    

Kingspan insulation
Polyurethane glue e.g. Titebond, Gorilla Glue etc
Glass-fibre tissue, Resin & Catalyst
Soil, coloured sands, fine grit, cement colourings such as Sealotone.

Goggles and Dust Mask
Electric Drill with circular wire brushes
Dremel with mini wire brushes
Paint Brush
Fine Sieve

Ask local builders for any left over off-cuts of Kingspan (high density foam insulation). Alternatively buy sheets of 100mm or 4 inch thick Kingspan and use that.

Remove the foil backing; dampen one surface as moisture is required for the glue to cure. Glue the sheets together with polyurethane glue ‘till you have a large enough block for your design. Remove any glue from hands immediately with thinners as otherwise it turns black and takes days to remove. It takes about 4 hours to set.

Before you go any further, consider how it is going to be displayed, standing on its own or in a pot? How will you secure it? Some strong anchor points that will approximately line up with holes in the pot, without blocking drainage, may be needed. Creating anchor points for wiring is discussed later.

The block is easily, but messily, carved with a wire brush in a drill, then a Dremel with a small wire brush for final detail. Be safe and wear a mask and eye protection when carving. The whole thing is then coated in tissue fibreglass mat. Mix small quantities of resin and catalyst so that it doesn’t go off prematurely. Working in warmer temperatures accelerates the hardening. To preserve the carving details, stipple on the resin/catalyst mix on a cheap (throw-away) stiff bristled paintbrush.

Work outdoors, or in a well ventilated area, and wear a gloves and a mask if you are sensitive. Cut the glass tissue into approximately 4" squares. Mix a small amount of resin. Stipple on the tissue, altering the angles and overlapping the edges. By using small squares, you can stretch the fabric to fit into crevices and over curves.

Allow the structure to cure overnight.

For large trees, more load bearing capacity may be required. Greater structural strength can be achieved by increasing the number of layers of glass mat. A coarse woven rovings layer or chopped strand mat before the fine mesh may obscure some carving detail, but the finished strength will be able to support heavier trees.

Use coarse woven rovings or chopped strand mat to create tabs around the base for wiring in to the pot if needed. Drill them after the resin has cured for 24 to 48 hours.

A stronger alternative for the base is to lay up a ‘slab’ built onto the bottom of the rock, in the intended pot. Use a release agent then after the resin is cured, lift it out and drill large holes corresponding to the drainage holes of the pot. This can be wired in and, when added, the soil and roots will provide extra firming 

To create a natural finish, bake a sandy loam in the oven, apply another coat of resin and sieve the loam onto it while still hot. To create different tones and colours use different ground materials, cement pigments etc. A fine sieve, that allows only small particles through, eliminates the shine that would otherwise mar the finish.

Another way to get a natural look is to stipple three or four colours all over in blotches-grey, brown, grey-brown, light grey brown, etc then let it dry. Then dip a wide brush into a little of each colour and splatter it on the rocks with a strong downward motion to distribute droplets of the colour finely on the surface. The stain splatters better if you thin it slightly first. Repeat with each colour, so that the colours mingle and give a natural look. Mix the darkest colour very thinly and apply with a small brush, carefully, in the crevices. This artificially increases shadows, adding an illusion of greater depth.

To encourage ‘aging’ of the ‘rock’ I use a mix of ground dried moss and yoghurt painted on, then mist regularly with water and later spray regularly. The ‘rock’ will weather and green up, giving it a much greater look of authenticity.

Some sources of materials and costings.


Fibreglass supplies  http://www.resin-supplies.co.uk/Pricelist.htm
GLASSFIBRE TISSUE 10 m ² pack pack @£16.50
Polyester resin (general purpose lay-up) 1 kilo pack + catalyst @£7.95

Titebond Polyurethane Glue £7.19 Axminster

Deva House, Knutsford Way, Sealand Industrial Estate, Chester
PHONE: 01244 376764

Ebay Gorilla Glue 8oz £11

Strong - One ounce covers one square foot. 100% Waterproof, bonds wood, stone, metal, ceramics, styrofoam and more. It foams 3 to 4 times its applied size, so a little goes a long way.

Indoor/Outdoor 100% Waterproof - Can be used indoors and outdoors, is unaffected by hot and cold temperatures and doesn't break down when exposed to rain and snow.

Stainable and Paintable - leaves an invisible glue line that can be painted and stained.

Easily Sanded - Gorilla Glue doesn’t gum up sandpaper or dull cutting tools.

Long Open Working Time - an open working time of 15 minutes. The glue remains slippery during assembly.

CFS Catalogue :LOW COST RESINS:Low Cost Resin

pack Low Cost Resin :      £12.74 (Excluding: VAT at 17.5%)

10 Square Metre Roll Surface Tissue 30gm Price:   £7.87 (Excluding: VAT at 17.5%)
1kg Roll Chopped Strand Mat  300gm  3.3 square metres  Price:   £3.86 (Excluding: VAT at 17.5%)

Sealotone Black, Brick Red, Brick Yellow, Brown,  Buff, half Kilo pack is £5

All Text, Diagrams & Photographs © Kevin Bailey 1998 - 2008