I don't think there is a quick answer to this! I can only speak from my own experience and impressions on the subject, and I agree with Prospero that the subject is vast, and there are many different methods, but it is a subject I like so I can go on and on about it
I think the main difference between the frame restoration work in many museums and the typical picture framer who offers restoration is the balance an average framer needs to make between being commercial, cost effective, while also offering a good quality service. There are of course picture framers who restore frames to the same level as museums, they themselves would work for high-end museums, private collectors, and dealers, but if talking about a typical framer than I can give an idea of some of my methods.
I think that museums are able to spend much more time on each frame, and the methods and materials used are from a strictly conservation point of view. Whereas your typical framer has to often do the work within a budget while still being as ethical as possible regarding the correct treatment. For some more detail on museum methods of restoring antique frames I can send you PDFs and links.
I offer a number of different solutions and price options, my frame restoration work is split between trade customers (art galleries and dealers), collectors, and the general public. I would say I am very much in the middle when it comes to framers who offer restoration, by that I mean it is not the main part of my business but it is an important part, I do more restoration than some but not as much as others.
Broadly speaking the trade customers require the work done very quickly, at a reasonable price, and to a good standard, for the collectors often the most important factor is very high quality work, the general public varies greatly from as inexpensive as possible to high quality. A lot will also depend on the quality of the frame, which will dictate what work is acceptable or not.
After I have got an idea from the customer of what they are looking for and the level of work they want, my usual method of restoration is as follows:
First I would write up a detailed report on condition, take photographs, and write a restoration plan to show the work I would carry out and also to help cost each process. This process is usually omitted for my regular customers as I know what they want and they know what I do, or for very simple jobs like replacing a small area of damage.
Each individual frame would present different problems, many will have been overpainted, this paint will have always oxidised and gone a browny bronze colour. Some customers (those on a budget perhaps) will not mind this look and will just want some missing ornament or damage repaired, but often this paint will have to be removed. This can often be done with acetone or other solvents, or in extreme cases paint stripper. The original gilding (or the second gilded layer of oil over the original water gilding) will often be in relatively good condition beneath the later applied paint. Water gilding will usually be unharmed by the use of solvents although any original oil gilding is more likely to be lifted if you are not very careful and meticulous.
Frames that have not been overpainted may have had some touching up with gilt paint, this is removed, any previous repairs, that are really bad are often removed. In some cases none or very little cleaning of the gilding will be required, in other cases the frames can be covered in thick dirt which is carefully cleaned. This can be very time consuming and painstaking, the cleaning can be done with a neutral soap like Vulpex or Swarfega, saliva, and numerous other liquids. Cleaning in museums and some commercial restoration studios is often done with triammonium citrate. In some cases the cleaning process is perhaps the most time consuming, it can be simple and straight forward or it can take great care and be quite difficult.
The surface of the gilding will often be fragile and the composition ornament will be loose, traditionally the surface would have been consolidated with a similar adhesive to what was used to create the frame, such as rabbit skin glue. I regularly use Paraloid B72 to consolidate loose or flaking gesso, it is applied in a solvent which has the benefit of penetrating deeply into the surface gesso and wood. The problem with the adhesive RSG is it's water based which can damage surrounding water gilding and is virtually impossible to remove without damaging/removing the original gilding. I have also started using Plextol B500 to consolidate and strengthen composition ornament. These and other relatively new methods of frame restoration are the result of research by museums, and leading restorers. I am simply learning about these methods and applying them to how I restore frames, and they are taking the place of more traditional materials and methods.
To make moulds of existing ornament you can use all sorts of materials, I mainly use a two part material called Otoform and Lab Putty. Many of these moulding materials are made for medical or dental work.
There are others I use/have used, and really there are loads of options available, such as: Vinamold, Steramold, Gelflex, numerous vinyls, and silicones, Capa 650, Kemdent, alginates, and so on, all with various properties.
Next comes the replacement of any missing composition ornament, carving, or gesso. The best material for replacing moulded ornament is still composition, although I use a lot of a two part epoxy putty called Magic Sculpt, especially for the majority of my repairs which are small areas of ornament, I also sometimes use Tiranti or a similar plaster. Large sections of carved frames really should be replaced with more carved wood. Gesso can be replaced with a number of different gesso mixtures.
Will continue with my method tomorrow.
