Painting on Location
by Donald A. Jusko

Fresco Mortar Hardness Tests


Testing in the top big box to see if alum will harden the lime and crushed limestone.
The smaller box to the right is without alum.
Top big box has lime, crushed limestone and alum.
Lettered "LIME MILK + BLACK for vine black and lime milk
Lettered "CASEIN" for vine black and casein.

Testing bottom big box to see if alum will harden the lime and sand mix from Zecchi.
The smaller box to the right is without alum.
Bottom big box has lime, sand and alum.
Lettered "LIME MILK + BLK" for titanium white or vine black and lime milk.
Feb. 8-04, the black is getting lighter.
Lettered "LIME WATER + BLACK" for black and lime water.
Feb. 8-04, maybe no change, Feb. 24th. no change.
Lettered "CASEIN" for vine black and casein.
Feb. 8-04, no color change.

Adding alum gives the lime a vinegar smell.
Testing lime water with vine black and lime milk with vine black.
The small separate box in the middle left test is 1 lime, 1 limestone crushed, 1 alum.
I wrote the word CASEIN in casein and vine black.

Feb. 8-04
I think the mortar with the alum is getting harder, but I can still nick them all with my finger nail.  
Feb. 9-04
I'm sure of it now, alum makes the lime harden faster. A letter to Riverside Cement confirms that alum is added to cement to make it harden faster.

Alum hard test

Testing the Whiteness of Mortar by Adding Marble Meal 02-11-04
The top line has alum added testing hardness, less on the left, more on the right.
Ratio of alum to lime water, .30 oz. dry to 2 oz. liquid.
There are 5 whiteness and hardness sections.

More alum and lime tests

100% sand 0% lime meal. 75% sand 25% lime meal, 50% sand 50% lime meal,
25% sand 75% marble meal and 0% sand 100% marble meal.
Immediately I could tell the difference between with and without alum. The mortar gets harder right away.
The more marble meal the better the mortar works while applying it, and the better the paint's application.
If you want to slow the drying time add 1% rabbit skin glue.
Day 2, the mortar with the most alum sings when you scratch it!
02-24-04, all tests are dry and hard. No alum is the softest but still hard.

So it's a quality thing, using crushed lime instead of sand will give you a white background to paint on. Even though the lime in a sand mortar mix will turn the background white when it's dry. To me color is everything, so I would much rather paint with a white background and lighten my tints with titanium white.
What I see is what I get.

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