What We Do
Consolidation of friable lime-based plaster using acrylic resin as a pre-wetting agent and an acrylic resin-based-adhesive as a bonding agent is a process that Historic Plaster Conservation Services (HPCS)
has been developing over the last twenty years.
Different situations present different opportunities, but the common element in all applications of our process is that the porous plaster is infused with resin solids that rebind it. Once consolidated, issues of the adhesion of the plaster to its sub-strate are addressed. One of several variations of an adhesive formulated from the same acrylic base is used to re-secure the plaster.
Plaster Conservation Process Description
The steps involved in the our plaster conservation process are more or less as follows.
- Investigate to determine the strength of the plaster as a system, and to determine if there is imminent danger of the loss of plaster due to collapse.
- At the plaster surface, analyze any pattern of deterioration.
- Determine the cause or causes of the problem. This is especially important if there is continuing trauma.
- Collect samples and test to determine how well the plaster will absorb resins and at what dilution resins will best be absorbed.
- Analyze the site situation to establish what conditions will prevail during any work.
- Address all occupational health and safety issues at this point.
The actual process depends on circumstances, but we often are in an attic situation where access to the back side of the plaster is not possible until rubble, insulation, soot, or temporary flooring is removed. This work must be done with utmost care, as there is at this point no real determination of just how fragile the plaster is in various areas. We usually recommend that Historic Plaster Conservation Services carry out or at least supervise this work in order to maintain a serious work attitude that gives due concern for the very high cost of any sort of error in judgment. It is usually very dirty work in terrible conditions, but requiring due diligence and concentration.
At some point, usually at the very outset, temporary ventilation is installed, both for the comfort and safety of the workers and to speed the evaporation process that is required to coalesce the resin that will be used. Ventilation requirements vary greatly from project to project.
After cleaning, assuming we have made the decision to consolidate an area of plaster and re-adhere it, we take the following steps:
Determine that there is no rubble between the lath and the suspended plaster.
Spray-apply the selected resin in dilute form with methanol. Usually between four and six applications are made. The material is applied in multiple coats on top of one another while still wet. Each successive application contains an increased concentration of resin solids until, on the final coating, a neat coat of the resin is applied. Sometimes a dye is used to tint the resin to aid in establishing a uniform application of coatings.
In this scenario, it is assumed that access to the face of the ceiling is provided and that a tower of rolling aluminium scaffold is erected for our use. From this vantage point, and in radio or direct contact with the technicians working above, we monitor the penetration of the resin into the plaster, and deal with any drips or leaks. Various methods are used to determine penetration. Depending on the relative temperatures between the attic and the ceiling surface, we can tell a good deal by monitoring the temperature change at the ceiling caused by the latent heat of evaporation.
RE-ALIGNMENT OF PLASTER WITH LATH
After the consolidation phase of the work is complete, any shifting of position that is desired can be undertaken. For the first several days, there is enough flexibility in the resin to allow the small movement required to re-align the ceiling planes.
RE-ADHERING PLASTER TO SUB-STRATE
The final phase of the work consists of the application of the selected adhesive to selected areas. The adhesive is made of the resin + lime + micro balloons + fluid coke. We have created an adhesive that expands on application to compensate for the loss of volume due to evaporation. In addition, it is thixotropic - that is, it changes from being a jelly to being a liquid with the application of stress (gentle vibration applied with a palm sander) and returns to its gel state when the vibration ceases. This property makes it penetrate fine cracks and lesions very well. It is tough and hard on setting, with very good tensile strength. It is non-toxic and non-combustible, and does not appeal to insects or rodents.
This process is sometimes as simple as coating the entire area with adhesive having the consistency of thick cream. In other situations, where plaster is suspended below the lath and has not been re-aligned, a slightly more elaborate concoction of adhesive is used. Here we want re-adhesion but we donít particularly want to put a large volume of heavy material in on top of the plaster. A thickened adhesive is used, and in some cases, stainless wires are embedded in the plaster with a resin grout and tied back to the wood lath.
In situations where the flat planes of a ceiling appear to have settled away from adjacent cast plaster decoration, a thorough inspection of all the connections of the castings is recommended. It is quite common to discover fastening lugs that are completely corroded and are not functional at all. In other cases there is unnoticed settlement of the casting, because it is not as severe as that in the flats. Inspection and, if required, maintenance of cast elements should be carried out during any ceiling conservation project.
The resin used is water-soluble acrylic, non-toxic and virtually harmless if ingested or splashed on the skin. The adhesive contains materials that in their unmixed state require the use of dust masks for safety, but that when mixed in the liquid adhesive present no safety or toxicity problems. The resin is diluted in methanol. Methanol is a hazardous material and appropriate precautions are taken for the safety of the workers exposed to it. On completion of a project, all the methanol used will have evaporated out of the plaster and will not be present in the building.
SAFETYHistoric Plaster Conservation Services Limited
is proud of its perfect safety record. Employees and other building occupants are protected during projects by sophisticated monitoring devices that measure the quality of the air in the atmosphere we are exposed to. When large amounts of the organic solvent methanol are used, employees are equipped with a supplied-air breathing apparatus (SABA) system. All employees are trained in first aid and all have WHIMIS training for the materials handled. Attic sites are routinely inspected for the presence of asbestos and bird droppings, with appropriate measures taken if such substances are found.
LIMITED SERVICES PROVIDEDHistoric Plaster Conservation Services Limited
does not undertake re-plastering projects, and is not in the business of reproducing plaster ornamentation. Our goal is to work with clients who value the original building fabric and wish to retain it. In unfortunate cases where some plaster has already collapsed, there is no better model for replication than the remaining plaster consolidated and secured to the structure.
The processes described here are not reversible, but are presented as permanent solutions to a problem that cannot be adequately dealt with using traditional materials. HPCS is fully aware of the issues and theory contained in the Venice Charter and other broad statements of conservation and restoration principles. We refer the skeptical reader to Article 10 of the Venice Charter.
Article 10, adopted by ICOMOS in 1965 "Where traditional techniques prove inadequate, consolidation of a monument can be achieved by the use of any modern technique for conservation and construction, the efficacy of which has been shown by scientific data and proved experience".
We believe that the traditional mechanical refastening technique using washers and wood screws through the plaster from the face is a flawed and unscientific approach. In many cases where this method has been used, deterioration continues until the ceiling once cherished for its decoration is grossly defaced by the presence of the washers and cosmetic over-painting to hide them.
We believe, especially in cases where the ceiling is noted for its decoration, that hierarchically, the plaster is less important than the decoration it carries. In these cases, the need to preserve the decoration without defacing it invites the use of our consolidation technique.
Historic Plaster Conservation Services Limited has completed many impressive projects over the last twenty years. None has ever been the subject of any kind of delayed failure or breakdown of the materials used. Our technical support from the resin manufacturer Rohm & Haas has always been excellent. We are always prepared to carry out special tests to ensure a safe result.