Workshop ‘Materialen en methoden voor oppervlakte reiniging en het verwijderen van film vormende materialen’. Deze driedaagse workshop werd gegeven door Paolo Cremonesi en was georganiseerd door Nora van der Veer. Het vond plaats in het Teylers Museum op 13-14 en 15 november 2019.
The oldest museum in the Netherlands, the Teylers Museum in Haarlem, welcomed us warmly on a grey November morning. Early birds, attendees thirsty for knowledge, came again to the same place from all over Europe. An enthusiastic atmosphere spread a good feeling in the air for the second day of the workshop lead by dr. Paolo Cremonesi. In the course „Emulsions for Surface Cleaning and Removal of Film – Forming Materials” during the second workshop day, we continued to gain knowledge from the first part entitled: The Aqueous Environment.
For a starter, our course leader gave us information about Complexants and chelators. Lecture was intense, but well prepared and comprehensible. To summarize this part I highlighted some of the most important information.
Complexants have one dent, consequently they are weak. Chelators have more dents, which makes them stronger. Complexants and chelators are potentially active on mineral pigments. Mostly on pigments such as: copper, iron, and magnesium.
The main Complexants we can list are: ammonia (stronger one) and amines such as Triethanolamine or Tris base.
The main ionizable chelators mentioned:
- Citric acid a tetra – dentate chelator,
- EDTA (Ethylene diamine – tetraacetic acid) an hexa – dentate chelator,
- DTPA (Diethylenetriamine – pentaacetic acid) octa – dentate chelator.
Important notice is that chelating power starts at pH 4-5 and is highest at pH 8-8.5 when citric acid is fully ionized. There is no chelating action in pH 2. Likewise, there is dependency: the more charges we have, the more chelators we got too.
During the lecture two types of solutions intended typically for surface cleaning has been presented to us:
- To make a pH 6 buffered chelator solution we can use citric acid. Citric acid can be buffered to pH 6 with sodium hydroxide. This solution will play two roles: buffer and chelator.
- To make a pH 8.5 buffered chelator solution suitable for removal we need 2 components: citric acid as a chelator and a proper buffer for pH 8.5, for example sodium diphosphate + sodium hydroxide.
- Alternatively, we can use DTPA with sodium hydroxide. In this case DTPA will play 2 roles: buffer and chelator as well.
Agarose gel disks for measuring pH
In the meantime dr. Cremonesi showed us how to prepare agarose disks for measuring pH from a surface, using special pH meter dedicated for this type of measuring.
- Firstly, checking hydrophilic or hydrophobic surface with a droplet of demineralized water. When the droplet doesn’t sink quickly into the surface, and keeps its regular shape, we deal with a hydrophobic surface. When the droplet is spreading, sinking quickly, the surface is more vulnerable for water.
- Preliminary the sensitivity of the surface to water mostly depends on the used pigments, whether they are mineral or organic,
- If the surface is hydrophilic, we need more rigid gel.
- Secondly, preparation of Agarose Gel disk for measuring pH.
0,3 in 10 ml – 3% for normal surface (hydrophobic tendency)
0,5 in 10 ml – 5% for sensitive surface (hydrophilic tendency)
We can heat it up in the microwave or in double pots (au bain marie).
Important: do not touch the agarose gel with metal; we don’t want to make a lot of ions. Also it is good to know that agar (not agarose) is ionic so we cannot use it for pH measuring.
- Applying of Agarose Gel disk.
- Well prepared agarose gel disks have a thickness of 2 mm.
- Apply on surface with the more dry side of the gels
- Wait for 2 minutes
- If the surface is super soiled, put the gel disk on 2 spots, with and without grime
- If we have a more hydrophobic surface, we should keep the gel longer, about 3 minutes.
- pH meter checking.
After processing we put the agarose gel disk into a Sushi pH meter. Turn it on and check the pH.
Surfactants: SURFace ACTive AgeNTS. Added to a liquid, surfactants decrease its surface tension and modify surface properties. Surfactants can modify water with ionizing them. There are two types of surfactants: ionizable, ionic and not ionizable.
Ionic surfactants –
(-) charge anionic alkaline
(+) charge cationic neutral / alkaline
(+) and (-) charges; amphoteric neutral acidic or alkaline.
Not ionizable –
Neutral, no charges compatible with all pH’s.
Generally, we are interested in nonionic surfactants, because they are safer for conservation treatment. Important information is that soaps are basically ionic surfactant. It’s worth remembering that alcohol surfactants are the safest surfactants so far.
Next information is that all surfactants are both hydrophilic and hydrophobic. However, these two characters are not 50 – 50%. Some surfactants are soluble in water some are not, but they can be soluble in oil. To express and predict the solubility of surfactants we use HLB Number (hydrophile – lipophile balance).
With reference to the workshop materials prepared by dr. Cremonesi, I will quote a relevant list of commonly used ionic and nonionic surfactants in conservation and restoration treatments.
In this issu – Gelling agents – were classified 3 main points:
- direct – viscosity increases as the material dissolves into the liquid
- requiring “chemical” activation and for example neutralization reaction by a base
- requiring “physical” activation. A heating or cooling process/cycle.
In gels issues we are also specifying non-ionic products. Cellulose ethers are non-ionic, they do not modify conductivity and pH. Different products are available with different hydrophilic / lipophilic character and different solubility’s.
Products derived thereof:
Methylcellulose, MC – Methylan, Glutofix, Methocel
Hydroxyethylcellulose, HEC – Tylose
Hydroxypropylcellulose, HPC – Klucel
Let’s take a closer look at KLUCEL. There are different types of KLUCEL available, such as: type H, type M, type G, type J, type L, type E. From them we can point out types E, L and J as adhesive, good concentration and low viscosity. It seems that type H is the best type of KLUCEL. Less than 2% of product is obtained for good viscosity. That means also that we don’t need to use long rinsing process to remove this type of KLUCEL.
In every type it is important not to overheat KLUCEL, not more than 40 degrees Celsius.
Xantham Gum is a polysaccharide, naturally produced by the bacterium. Another product name for XANTHAM GUM is VANZAN NF-C and other types as well. XANTHAM GUM is ionic and acidic. This type of gel is pseudo – plastic, in other words thixotropic. This means that it is more easy to work on surfaces, less adhesive than cellulose ethers, and easy to clear from surfaces. Viscosity is also stable up to the higher temperatures. It’s a good polymer for making emulsions.
Carbopol and Pemulen
Carbopol and Pemulen are Polyacrylic acid derivatives. These polymers are ionic and acidic. They require a base for gelling. These products have a very high viscosity and high retention of water gels, even at low concentration. They contain good stability to bio deterioration. Worth knowing is that polyacrylic acid dispersed in water, form non-ionized macromolecules, but it is not acting as a gelling material.
Bases for neutralizing polyacrylic acid derivatives are: triethanolamine, fatty ethoxylated mines such as ETHOMEEN, and inorganic hydroxides such as sodium, potassium and ammonium.
For CARBOPOL ULTREZ 21 the maximum viscosity we can get is 5,5 pH.
Agar and agarose
These products are polysaccharide gelling agents. They are thermo – reversible rigid gels. Agarose has a linear chain, is nonionic neutral polymer composed of galactose. Agar has a complex structure, containing polysaccharide extracted from algae.
Agar is ionic, in contrast to nonionic agarose.
Lunch Teylers Museum
Cleaning Yellowish Varnish
In this case we used 2 different types of cleaning systems on the same painting. Firstly, we used a macro emulsion with surfactant on a less yellowish varnish part and Carbopol gel on a more dirty varnish layer.
1) Macro emulsion
+ 10 ml – pH 5,5 – water (aqueous base)
pH 5,5 – (without surfactant)
+ 4 ml – Ecosurf EH-6
Mix it firstly with battery charged blender.
And + 90 ml of hydrocarbons (ISO – octane in this case, can be Shellsol D40 also)
But with isooctane we are pouring by 10 ml portion, small amount and mix it, small amount and mix it and so on. Stir it.
Good to know that this emulsion is not suitable for acrylics because it includes surfactant.
For cleaning acrylics, we are using only buffers and chelators, without surfactants.
2) Carbopol + benzyl alcohol
+ 7g of gel
+ 5% BA (0,35g)
Result: very good cleaning
Rinsing water prepared for 8 pH
- Sparkling water (pH 5,3) + ammonium water (1ml in 100 ml of water)
Mix it all, till pH will be proper for our expands.
Cleaning Yellowish Varnish 1
Cleaning Yellowish Varnish 2
First of all, we made, based on acetone, tests for cleaning soiled gilding.
Start cleaning gilding with:
- Acetone + isooctane in various proportions – no results
- Acetone + ethanol mixture – no visible results
Thereafter, we prepared silicone gel for cleaning. Used for that silicone gel – VELVESIL gel.
+ 5 g KSG gel – VELVESIL
+ 0,5 g of benzyl alcohol
Gel starts becoming milky white.
Rinsing with :
- Shellsol D40
Good result, gilding is renovated, shine is restored.