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Stone cleaning:All you want to know about laser cleaning stone

pulse laser cleaning

Table of Contents

Commonly used stone cleaning methods and problems

At home and abroad building stone or stone artifacts surface pollutant removal technology has chemical cleaning, steam jet cleaning, particle jet cleaning, laser cleaning stone, etc.

In a variety of stone or marble cleaning technology,The chemical cleaning stone problem is difficult to identify the residue on the long-term impact of the stone, in addition to environmental pollution. The problem of steam jet stone cleaning technology is that the surface has deteriorated:The problem of steam jet cleaning technology is that when it is used for stone with deteriorated surface, the high temperature of steam will cause uneven thermal expansion inside and outside the stone, resulting in stress damage. Particle jet stone cleaning technology: The particle jet cleaning technology will seriously wear the surface layer of the stone, especially some angular parts. Laser cleaning stone technology: the principle is to use high power density of the laser beam irradiation of the stone surface, so that the surface of the pollutants occur instantaneous fragmentation, evaporation or stripping effect, Laser cleaning stone characteristics is good cleaning effect, non-contact, non-polluting environment, no residue, safe and reliable, water-saving, can be precisely controlled, wide application, low operating costs, especially for the surface of fine stone carvings,stone laser cleaning advantages are many traditional cleaning process is not. Can not be compared.

Stone carvings and other age-old high-grade stone artwork, due to its fine and fragile surface structure, become the earliest application of laser cleaning field. As early as 20 years ago, the British JohnAsmus with ruby pulse laser cleaning machine successfully cleaned the weathered crust layer on the marble carving. People found that the use of laser to remove dirt from the surface of stone artifacts have unique advantages, it can be very precise control of the beam on the complex surface movement, remove dirt without hurting the heritage stone.

100w pulse laser cleaning machine

Pulse Laser Cleaning Machine For Stone

The difficulty of stone cleaning

The bonding force between the dirt and the stone surface is mainly physical and weak chemical forces. Weak chemical forces including hydrogen bonding and charge transfer formation of bonding energy, etc., physical forces including Van der Waals forces (including electrostatic, induced and dispersion effects) and capillary forces. Stone cleaning is more difficult than other hard surface materials cleaning method, that is due to the presence of a large number of natural stone microporous, microporous capillary forces not only make the dirt and stone between the various bonding forces have been enhanced, while its wrapping effect also makes a variety of cleaning external forces difficult to play.

Laser cleaning machine in the stone industry application principle

Stone Laser cleaning process

Laser leaning stone is workable for all kinds of stone, such as bricks, marble and graffiti removals. Laser is a monochromatic and directional light radiation. The laser beam can be focused by a combination of lenses to concentrate the beam into a small range or area. The laser beam can produce at least three effects.

(1) Laser beam will produce a mechanical resonance phenomenon on the solid surface, causing the surface dirt layer or condensate to break off.

(2) Laser beam will cause the surface dirt layer to expand thermally, thus overcoming the adsorption force of the substrate material on the dirt particles and detaching them from the surface of the object.

(3) in an instant so that the dirt molecules evaporate, vaporization or decomposition.

Comprehensive, the laser cleaning stone technology is the use of ① laser pulse vibration; ② particle thermal expansion; ③ molecular photolysis or phase change of the three effects or their combined action to overcome the bonding force between the surface of the substrate material of dirt, so that it is removed from the surface of the object and achieve the purpose of cleaning.

Two kinds of laser cleaning stone classification.

According to the analysis of the optical properties of the matrix material and dirt being cleaned, and laser cleaning stone or laser cleaning marble mechanism can be divided into two categories.

One laser cleaning stone is the use of the matrix material and the surface attached to the dirt on a wavelength of laser energy absorption coefficient difference, so that the laser energy is fully absorbed by the attached dirt, thus heat expansion or vaporization volatilization, and the vaporization of the formation of vapor flow driven away from the matrix material and achieve the purpose of cleaning. The requirement is that the absorption coefficient of laser energy of the bricks, marble and graffiti removal material to be small, so that the substrate material is not damaged. Therefore, the key to achieving safe and efficient cleaning is to choose the appropriate laser wavelength and control the energy density.

Another category is for the substrate material and surface attachment to the laser beam absorption coefficient difference is not large, or surface attachment to the heat will produce toxic substances and other cases of cleaning, is usually the use of higher frequency and power of the pulse laser impact to be cleaned surface, so that part of the laser beam into sound waves, sound waves in the impact of the lower hard surface after the return, the return part of the incident sound waves with the laser generated interference, thus Generate high-energy resonance wave, so that the scale layer of tiny bursts, resulting in crushing, easy to detach from the surface of the substrate material.

For the stone marble cleaning surface dirt, the above mechanisms are often used in combination. The laser cleaning machine frequency (0.5 to 30 pulses per second) and amplitude (8 to 25 ns) of the beam light pulse is usually adjusted according to the situation of the treated stone and dirt, so that the dirt material properly absorb the light energy. Pulsed laser constantly repeating the frequency of the impact effect can make the stone surface and microporous dirt loose, when the laser impact force is greater than the adsorption force of the substrate on the dirt particles, dirt particles from the substrate to achieve cleaning purposes. When the laser photon energy is greater than the (adhesion layer) dirt molecules bond energy, laser photolysis, light stripping effect, such as KrF excimer laser photon energy SeV, greater than the organic dirt O-O, H-H, O-H, C-C, C-H and N-H and other chemical bond bond energy, the action of the laser can destroy part of the chemical bond, decomposition of organic matter, thus cleaning organic oil stains.

When the laser beam energy density is further increased, some inorganic dirt, such as salts containing K, Na, and inorganic matter generated by the photolysis of organic dirt can be heated and expanded by the laser, resulting in thermal stripping (i.e., light stripping) and off the surface of the substrate. In order to make full use of the various effects of the laser, to enhance the cleaning effect of the laser, often artificially coated with some water or water and methanol or ethanol mixture of liquid on the surface of the substrate to be cleaned in advance. When the laser irradiation on the liquid film, the liquid film due to the rapid heat, will produce explosive vaporization. The impact of the explosion loosens the dirt on the surface of the substrate and flies away from the surface of the substrate with the shock wave, achieving the purpose of decontamination. In this method, although there is also the vibration of the dirt particles and the thermal expansion of the particles, but the role of explosive shock wave is the main, this method is also known as: laser + liquid film method. The thickness of the liquid film covering the surface of the substrate is generally l0Um.

Laser cleaning stone methods and examples

Analysis from the method, laser cleaning stone method has four kinds of.

Laser dry cleaning method

Laser + liquid film method

Laser + inert gas method

Laser + non-corrosive chemical cleaning method

  • (1) laser dry cleaning method for stone cleaning, that is, the use of pulsed laser direct radiation decontamination.
  • (2) laser + liquid film method for stone cleaning, that is, first deposited a liquid film on the surface of the substrate, and then decontaminated with laser radiation.
  • (3) laser + inert gas method for stone cleaner, that is, in the laser radiation at the same time, with inert gas blowing to the substrate surface, when the dirt from the surface will be stripped immediately after the gas blowing away from the surface, in order to avoid surface contamination and oxidation again.
  • (4) the use of laser to loosen the dirt, and then cleaned with non-corrosive chemical methods.

Currently, the first 3 methods are commonly used. The fourth method is only seen in the cleaning of stone artifacts.

Laser cleaning stone operation steps.

  • (1) To be cleaned to determine the threshold of material damage.The laser issued by the laser focus, so that the laser in the same material with the material to be cleaned on the surface of the non-polluted stone. Laser spot on the marble or bricks surface to be cleaned to form a laser spot, in turn, to increase the laser energy density, launch laser, irradiated stone material surface at different locations, when the material surface damage laser energy density is the same material with the material to be cleaned on the surface of the uncontaminated stone,The value of the damage to the material to be cleaned, that is, the value of the damage to the material to be cleaned;
  • (2) the use of the same laser in step (1), with a laser energy density lower than the damage threshold, irradiation to be cleaned building stone or stone artifacts surface, the cleaning area with the surface of the object to be cleaned without staining part of the comparison, to determine the cleaning effect results, and then remove the pollutants to be cleaned in turn on the surface.

Marble Stone laser cleaning application cases.

Stone Laser cleaning of the best application case is September 1992, the UNESCO-owned World Heritage Protection Organization to commemorate the 20th anniversary of the organization, the implementation of the very famous British Amiens Cathedral repair project. The very fine marble carvings on the west side of Amiens Cathedral’s Notre Dame Gate were the key to the project. During the year-long repair project, the laser beam was used to remove the black layer of dirt covering the carved marble pattern, which was several millimeters thick, and the original color of the marble surface came out, bringing back the glory of the beautiful carving.

People use the electron microscope to observe the surface of the stone carving after laser cleaning, found that the structure of the stone did not change after laser cleaning, the cleaned stone surface is both smooth and flat, no damage. This is completely different from the surface after cleaning with micro-particle blasting method (sandblasting method). The damage to the marble surface structure after cleaning by the microparticle blasting method is difficult to avoid, especially for the marble surface that already has a sulfate scale layer. The observation of electron microscope also found that after laser irradiation, the various properties of the rock material under the surface are neither degraded nor changed. At present, after cleaning lime with laser, the work of marble and other high-grade stone material surface dirt has become a new promising business project. In addition to the cleaning of stone materials, laser cleaning in glass, quartz, metal, molds, teeth, chips, electrodes, magnetic heads, disks and a variety of microelectronic products have been very good results in the cleaning of the application.

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