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How to restore the stone?

In spite of its reputation, the stone is not eternal, especially when it is a limestone, exposed to the aggressions of a polluted atmosphere.
It is not a question here of restoring the arm of a statue, or the granite arch of a porch, but of repairing a break on a cornice or of giving back to the mantel of a chimney its old appearance.

Stripping the stone

As it is difficult to accurately assess the extent of possible damage under dirt, moss and lichen, any renovation operation necessarily begins with a general stripping of the object or the damaged item.

Cleaning the stone
It is a more delicate operation than one generally imagines, because it must be adapted to each type of stone.
Be very careful before attacking the stone exposed to bad weatherand especially acid rain, which transforms calcium carbonate (the basic constituent of calcareous stones) into calcium sulphate, that is to say into friable plaster.

  • Mechanical brushing with a rotating wire brush, is to proscribe on all the limestones.
  • A small rotating brush made of nylon thread however, it can be used to remove small patterns or de-ditch the bottom of grooves on a cornice.
  • The pressure washer is effective at removing a layer of mosses and lichens:
- use the sandblasting function with caution on soft stones: first try a not very visible part of the object to be cleaned;
- do not use the rotating nozzle which may irreparably dig the stone;
- use a spatula or a chewing knife to take off the lichens in the bottom of the reliefs.

The plaster of plaster
It is a very popular way for professionals in the restoration of sculptures, bas-reliefs and statues. This is the best way to obtain the complete removal of a stone object: it avoids as much as possible the risk of attacking the sick parties.

  • Pour demineralized water in a large plastic container, and dissolve 50 g of ammonium carbonate per liter.
  • Saturate the paper pulp mixture to have a homogeneous paste. This pulp is obtained by soaking, always in demineralized water, plain white paper (never printed paper).
  • Apply this paste (handle with gloves) on the stone to be stripped and let it dry.
  • Eliminate the dry pulp with quack grass brush.
  • Rinse abundantly under pressure.

Repair fragments and chips

After stripping and repair, apply a hardener on the renovated part.

Apply a hardener

Aggressions of time and men cause damage and sometimes damage the stone, or eliminate some details of a carved part. In some cases, the stone becomes friable. It is then necessary to intervene to restore the work, taking care never to disfigure it by a too visible repair. It should never be a question of making a restoration "to nine" anachronistic and always very visible. The patina of time must be respected, because it participates in the charm of the old.

Synthetic resins
Synthetic resins, added to single-component repair mortars, ready for use, are very practical for the simplest repairs. Wear rubber gloves to handle these products that may cause skin allergies, even burns.

Lime mortar
It is widely used by specialists to restore details on limestone statues. The lime flower (air lime extinguished) is the base of this mortar, added sieved sand, dyestuffs, marble powder and, for objects exposed outside, water-repellent resins.

Reconstitute a stone angle

Sometimes it is necessary to slightly tint the filling product so that it is harmonized with the rest of the stone.

Stain the filler

1. Prepare the filler and color it to give it the hue of the stone to restore.

A piece of folded newspaper makes it possible to mold an angle.

Mold the angle

2. Mold a sheet of folded paper on the corner and let cure 24 hours.

A coating knife can be used to grind both sides of the corner.

Use a coating knife

3. Apply the product on both sides of the corner, with a coating knife.

Make a piece of stucco cornice

The imprint of a good part of the cornice can be taken with modeling clay.

Take the impression

1. Using a talcated metal cylinder, make a strip of modeling clay 2 to 3 cm thick and mold it on the part to be redone.

The impression of model clay must be placed in a box on a bed of sand.

Make a mold

2. Unmold the modeling clay and place this shape in a box filled with sand that will prevent it from deforming, the two edges horizontal.

Once the mold is filled with modeling plaster, use hemp fiber in it.

Strengthen with hemp fiber

3. Pour some modeling plaster up to about halfway up the mold, then arm the piece with hemp fiber.

The filling of the mold can be completed by still drowning in the plaster some hemp fibers.

Complete filling the mold

4. Fill the mold completely and place some hemp threads in the edges of the widest part.

The repair part should be adjusted to the damaged area and then glued with adhesive sealant.

Paste repair

5. After drying (12 to 16 h), unmold and re-cut the piece so that it fits into the part to be redone. Glue it in place using a putty-glue, remove the excess and sand after drying.

(photos / visuals: © DIY_Prod, except special mention)

On the same topic

  • Questions answers
    • How to clean a stone chimney Gard?
    • How to clean and repair a stone fireplace?
    • How to restore the stone of a vaulted cellar?
    • Restore a table "imitation marble"
  • DIY tips
    • Renovate marble: the right techniques
    • Repairing face stones
    • Reconstitute cornices and stone frames
    • Repair a piece of furniture with putty or wood paste
    • Restoration of old, wet and saltpainted masonry
    • Traditional restoration of old masonry

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