does anybody know anything about adding borax and or alum to plaster paris to make it as hard as marble?

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aeray7 years ago
"Keene's Cement. When it is necessary to give walls, ceilings, etc. a hard and highly polished surface, a prepared plaster known as Keene's cement is generally used as a finishing coat. Strictly speaking, this is not a cement, but is made of plaster of Paris, soaked in a solution of alum and then recalcined. Applied to the walls, this material becomes very hard and takes a high polish, so that surfaces finished with it may be washed without injury. Its hardness also makes it very satisfactory to use for finishing the lower portions of walls where the surface is liable to injury by contact with furniture, etc."
afridave (author)  aeray7 years ago
excellent best answer thank you
discostu9563 months ago

Very old thread here, but might have some help for you or others searching for this. I have similar interest to yourself and found this question searching for answers. A result further down the page was this:

In there, the very mixture you are taking about was mentioned- equal parts alum and borax added to 20 parts water was what they said I think. Similar results to hydracal I think they also said. Now I need some alum!

seandogue7 years ago
Nope. I know that in the old days, when they plastered walls, the underpinning was a mixture of plaster, sand, and horse hair.

I've used fiberglass, pulled from standard insulation, as a strengthener in interior wall repairs, since horse hair isn't quite as plentiful as it once was.

I've also used latex concrete additive to strengthen plaster for the finish coat.. Your post is interesting, but where did you come up with those two ingredients, if I may ask?
afridave (author)  seandogue7 years ago
About 10 years ago i read a very old victorian book called 1001 things to do or something along those lines,and i remember reading something about them making outdoor statues out of plaster paris and adding something to make it "hard as marble" and waterproofing them.I cant remember what they added but i thought it was either alum or borax:
They do still make moldings for outdoor use using plaster, but I think a coating is what maintains their integrity, which is by top coating with linseed oil and then an oil based paint. Thanks for the tip.

I'll have to search around, maybe ask my mom if she has anything in her library that might illuminate me.
BTW, alum, is a pretty fascinating substance. I knew that it was used for crisping lettuce and other salad vegetables, in underarm deodorant, as a stiptick (sp) for shaving cuts (I still have a stick that I use from time to time when I have to shave with a "true" razor and for treatment of cancer sores (I used to suffer from them when I'd eat tomatoes, eggplant, and many type of fish until as recently as ~40 years old, although it seems to have completely dissipated, since I no longer get them - knocks on wood), but here's a link that will illustrate this often overlooked and once household chemical

Although they seem to have missed the use as a veggie crisper, which I've read in several old cookbooks throughout the years.
afridave (author)  seandogue7 years ago
yip billiant stuff ,i was a boy scout as a kid and remember using it for something,but for the life of me i cant remember what. which is unusual bieng as thet was only yesterday.
lol... yeah, I just learned how to ride a bike a few weeks back ;-) (44 years ago)

It's amazing how much we've forgotten in the past 40 or 50 years. Alot of standard household knowledge has been lost with the advent of "modern" methods. I'm still surprised about the lack of common knowledge about the powers of white vinegar, for instance...People go to the store and buy all sorts of cleaners that aren't any better than (and in many cases are worse). Mildew for instance. White vinegar is amazing and deadly to mildew, far more so ime than bleach.

I was fortunate to have worked for a lot of WWII vets and picked up several great tidbits from them and their wives along the way. My mom knows quite a few old tricks too and the occasional reference in a book or something learned here or some other website from our elders...
afridave (author)  seandogue7 years ago
yip my dad was a boy in britian during the blitz and that generation knew how to do stuff,dad was a carpenter,joiner,cabinet maker fix anythinger (as long as no electronics) and i tell you he was a genius,they really knew how to train em back then ,he was more of a design engineer structurao) than any youngsters (my age) ive ever met.and mom can turn a penny over and over and over again.
Sounds a bit like my dad. Math prof by day (now retired), excellent carpenter, auto mechanic, musician, and all around intellectual. I suppose when you grow up in the depression or times of War, you learn alot of things people don't in more prosperous times.

But in general, I think we're losing alot of very useful info due to the plastic wrapped consumer culture we've been indoctrinated to support during your and my generation's upbringing. Some or all of which we may very well need in the coming years, what with the teetering failure of the techno society.

(You're what, mid 40s???) I'm just a bit older. My dad was born in '28.
afridave (author)  seandogue7 years ago
ill be 44 in january. dad was born 34.Dad knew about everything,By no means all about everything but there was nothing that he could not enter into a discussion on and bring something worthwhile to the table.And a real sharp whitty sense of humor did he have too an a naughty sparkle in his eyes,the life and soul of all events he was involved in. unfortunatly hes gone now and all his wisdom too (thats the word were on about WISDOM)

yip consumerism sucks,you guys at least still have a non consumerist foundation,but here in the developing world they have rushed directly into it from foundation upwards,scary. when i tell people (intelligent people too) that i make cheese i get this......WOW out of milk? no out of sulfuric bloody acid ....what else is cheese made of?

but the techno thing is not all bad,were talking and i estimate you about 14000 km away,quite amazing id say.
but yes i agree taht a lot of the "information revolution" is a load of hype.Id prefer to call it the data revolution and lots and lots of the data is in no way informative but mearly a loead of crap

Ive read a lot of victorian stuff (hg wells is my fav) and you could go to the chemist down the road an by a pound of this and a gram of that and a bottle of whatsit called and make whatever you wished ,try it now,the chemist looks at you as if your from another planet.

thats because hes not a chemist at all,nows nothing about chemistry.hes mearly a drug salesman for a bunch of large multinational companies.

Burf7 years ago
No. Borax or alum added to plaster of paris acts as a plasticizer, it retards the curing process. It will not make it hard as marble.
You can add some strength to plaster of paris by adding white PVA glue (Elmer's glue) as you are mixing it but it is not going to get anywhere near the hardness of marble with either white glue or borax.
afridave (author)  Burf7 years ago
there is something too this hardening thing,after a bit of googling ive found a few discreet hints of what im looking for but have not narrowed it down to an actual recipe as of yet .i will continue to search.
I'd be interested to see what nylon fiber (concrete reinforcing fibers) and various glues/silicone products would do to improve the strength of plaster. Check out the Sugru Substitute Instructable.
afridave (author)  AngryRedhead7 years ago
I have attempted coating the face of the mold with polyester resin ,and plaster of paris bieng porous absorbs it and it then sets hard.This certainly does increase the strength of the plaster surface but it burns.
afridave (author)  AngryRedhead7 years ago
I am actually experimenting with the aim of increasing the durability and life span of intricate plaster molds into which i cast lead,ive got the two piece mold making almost to an art but it deteriorates too quickly.I know silicone type rubber is exellent for this but here were i live it is imported and way too expensive to be used as an everyday experimental material.
caarntedd7 years ago
Add cement.
afridave (author)  caarntedd7 years ago
ill try that, simple and straight forward,sometimes simplicity is so simple you dont even think of it.
orksecurity7 years ago
I don't think anything will make plaster as hard as marble. There may be ways to make it harder than it normally is...

Seems to me that if this works, websearching on some combination of those keywords ought to find instructions. And in fact Google Books does find an old reference to something like this, at
afridave (author)  orksecurity7 years ago
aeray7 years ago
I'm pretty sure that this is what you're thinking of, and is a good place to start your research (link at the bottom):
"Scagliola (from the Italian scaglia, meaning "chips"), is a technique for producing stucco columns, sculptures, and other architectural elements that resemble inlays in marble and semi-precious stones. The Scagliola technique came into fashion in 17th century Tuscany as an effective substitute for costly marble inlays, the pietra dura works created for the Medici family in Florence. Scagliola is a composite substance made from selenite, glue and natural pigments, imitating marble and other hard stones. The material may be veined with colors and applied to a core, or desired pattern may be carved into a previously prepared scagliola matrix. The pattern’s indentations are then filled with the colored, plaster-like scagliola composite, and then polished with flax oil for brightness, and wax for protection. The combination of materials and technique provides a complex texture, and richness of color not available in natural veined marbles. A comparable material is terrazzo. "Marmorino" is a synonym, but scagliola and terrazzo should not be confused with plaster of Paris, which is one ingredient."
afridave (author)  aeray7 years ago
Thank you ,i will certainly research and follow your input a little further.
aeray aeray7 years ago
Much of the "marble" in the Idaho Statehouse is actually made of this material, installed by a secretive family of Italian craftsmen.
lemonie7 years ago

Synthetic marble
is polymer-based. Marble is pretty hard as stone goes isn't it?