I've been "chatting" online with someone who makes his own cement. When I told him my basic recipe (boiled & raw linseed oil, whiting and color to suit, occasionally a splash of turps to help mixing as it seems to evaporate quickly) and said that I felt cement is best when it remains elastic rather than becomes rock hard.
He likes a similar mix but insists on adding plaster of Paris because it makes the cement "stronger". He also maintains that the lack of plaster will result in whiting drying out & becoming white powder. While I've seen this sort of powder in some old leads, I never thought it might be dried out, oil-less whiting.
He also contends that if plaster attracts humidity from the air then his open bag of plaster should be hard as a rock!
I thought the oil & whiting created some sort of chemical reaction which bonded the whiting and the curing oil. I also thought plaster needed water to set up and assumed oil wouldn't make it hard.
I feel fine about folks having their favorite recipes but I'm not chemically knowledgeable enough to be sure others who read our "chat" will get the best information or even the best foundation for forming an opinion. Can anyone here clear up at least the chemistry for me? I doubt that this person will change his mind but I'm very uneasy about leaving the science in doubt.
Your friend would need a VERY humid place to make plaster in the bag as you need about a quart of water for 2 1/2 pounds of dry plaster.
The issue is that dried plaster in the putty will absorb moisture and will leach (bloom) out onto the surface of the glass, allowing the possibility of mold and microorganisms to form that can damage the glass. Much like mildew forming on interior plaster walls.
The old school recipe that I was taught consists of Portland cement, painters black for color, and gasoline... it makes a cement that really works into the came nice with a brush, cleans up ok with whiting and hardens to a rock pretty fast.
With that said I NEVER use this recipe... at my studio we use premade glazing putty, we use Wonder Putty, we use it straight for installs requiring glazing as intended. But we also use it for all cementing of windows, I have an old paint shaker so I mix putty with some paint thinner and a little japan dryer, shake it till it's mixed together and very soft and use a brush to putty and clean up with wood flour rather than whitening.
This gives an elastic putty, not a rock hard one. After working on hundreds of restorations with all sorts of different puttys I have some opinions based on my experiences. I will always advocate for elasticy, softer putty. It makes for more forgiving install with less break risk, it allows windows to move and flex with the structure it's installed in throughout the year. Also not least important it makes for a MUCH better situation down the road if the window needs to be repaired or even for the next guy 80years from now restoring it. If you've ever needed to dismantle or repair a window that's been actually been "cemented" without breaking anything then you know how I come to these opinions.
Also I was taught that the white dust that you find within and old came is mostly broken down lead resulting in lead dust, this is one of the most dangerous things that we working in this field can encounter... it is not safe to assume that white dust is just old putty.
A couple of points
1- When the manufactures speak of "elasticity" of their glazing compounds they are referring to putty glazing of glass to sash. The putty is applied in larger (thicker) amounts then what we push under the lead came. It is also painted on the surface. Thus preventing air from "hardening" the putty. So I am not convinced that Wonder Putty is much, if any, better then linseed oil putty in stained glass work.
2- By adding Japan drier and paint thinner you are speeding up the set up time of the putty, thus reducing long term elasticity.
3- Old putty, prior to 1978 often times contained either red or white lead. This helped create a better bond to the glass. Also making the putty dust toxic when dried out.