There are some potential problems with incorporating newly made leaded glass panels into sealed units. But all the problems can be overcome by using different materials or different sizes of products.
The oxidation products from the putty fogs up the inside of the glass, so butyl rather than linseed oil mastic is required. As much as possible, repairs are simply fitted into the came and the leaves dressed down to the glass - the waterproof requirement no longer applies after all. In this way no new material is introduced into the sealed unit.
Most leaded glass windows need to be reduced in size to fit into sealed units. The amount of reduction is determined by the width of the spacers. The spacers that double glazing firms use are large - usually 25mm wide. However there are thinner spacers - 13mm wide - that can be used to reduce the amount of the window that needs to be removed to get it into the unit.
Most of the time, there is some kind of border at the edges of the window, or a continuation of design between adjoining windows that no damage is done to the image by small reductions. But just now I have a set of three windows that will not easily be reduced in size without damaging the design.
Fortunately, the clients agreed with me that the windows should be kept and that the sealed unit size should be altered to accommodate the glass. This does require the joiner to make new sashes and casements for this extension to the house. It also requires some adjustment to the size of the window opening to allow for the marginally wider window frames required. To reduce the amount of change, the leaves of the outer cames have been shaved to the hearts, giving about 5mm each side. The new sash frames are being made narrower to reduce the expansion required of the casements, and the mullions are being made a little narrower too.