A basic property of any gypsum plaster is its "normal consistency", or the amount of water required to mix a given amount of plaster to a state of fluidity. Both the gypsum plaster and the water are measured by weight, and the required amounts are expressed as a numerical ratio. A mixture of 1kg of water to 1.5kg of gypsum plaster, for example, can be expressed as 2 parts water to 3 parts plaster, or a 2:3 ratio. Usually, the equation is expressed in terms of 100 parts of plaster. The 2:3 ratio would then become 67:100. When 100 parts of plaster are added to 67 parts of water, the mixture has a particular consistency and this is expressed by the number 67 alone. The number 80 would express the consistency of the mix when 100 parts of plaster are added to 80 parts of water.
Consistency
As the number refers to the amount of water used per 100 parts of plaster, the higher the consistency number, the more fluid the mix will be. When less water is used and the mix is therefore less fluid, the setting time and the period of plasticity for the mix are comparatively short. When more water is used and the density of the mix is reduced, the setting time is lengthened.
The consistency affects not only the setting time but the hardness and the compressive strength of the set plaster as well, which in turn correlates closely with resistance to breakage and useful life. The higher the consistency number, that is, the more water required, the softer and weaker the final plaster that results. This is because plaster becomes solid by the formation of tightly interlaced gypsum crystals; as more water is added, these crystals are pushed further apart, thus making for a weaker structure.
A consistency number in the 65 to 85 range indicates that the set plaster will be of medium hardness. U.S. Gypsum rates the consistency values as follows: 94 to 77, soft to medium; 76 to 59, medium to hard; anything less than 58, hard to extra-hard. It is extremely important, therefore, to consider the ultimate use of the plaster mix not only in choosing the type of plaster to use but also in adjusting the water-to-plaster ratio for the chosen plaster. Remembering that as you increase the amount of water you lose hardness and strength in the set plaster, you must decide whether workability (such as carving) or strength (such as longer mould life) is more essential for your purpose.
Water-to-plaster ratio
The water-to-plaster ratio is important in another respect as well. When proper proportions are being mixed, the gypsum plaster gradually reaches a creamy state, becoming opaque when seen on the hand or stirrer. It is not thin and watery. When it has reached the opaque stage, it is practically ready to be poured. One can be misled, however, if the water-to-plaster ratio strays too far. If too much plaster is present, the mix will appear to be creamy and opaque in a much shorter time; but when poured it will sit with water on the surface and take an extra-long time to set. This results in a non-hom*ogenous piece - the plaster will be porous on top and hard on the bottom. If too much water is present, the mix will take an extra long time to reach the creamy stage and then, all of a sudden, it will set overly fast. The piece will still have good hom*ogeneity, but the set plaster will be softer than it would have been had the desired ratio been used. In either case, after you have gained experience and have become acquainted with the material, you will achieve a "feel" for the plaster; you will be aware of any imbalance in the mix early enough in the mixing process to remedy it by adding a dash of one or the other ingredient to bring about the balance.
It is advisable, then, to mix several small batches (approximately 1/2kg) of the plaster you have chosen for your particular work, until you have acquired a little of the feel of a proper ratio. Learning to recognize the various stages in setting as the changes occur, the precise consistency to use, and the proper time to pour or shape any particular plaster can best be determined by your own experience with it, since many variables in shop and studio techniques influence the results.
FAQs
When more water is used and the density of the mix is reduced, the setting time is lengthened. The consistency affects not only the setting time but the hardness and the compressive strength of the set plaster as well, which in turn correlates closely with resistance to breakage and useful life.
What is the consistency of plaster mix? ›
The texture should be creamy, able to flow but not runny. It should not be so stuff that it creates peaks. Remember that as soon as the powder is in the water the setting process has already started so mix promptly.
What is the ratio of water to plaster? ›
The water/plaster ratio is referred to as consistency. It is expressed as parts water per 100 parts plaster. Typical consistencies for Pottery Plaster (K60) range from 60-75 (60-75/100).
What is the importance of water powder ratio? ›
The water/powder ratio is considered a determining factor of the physical and chemical properties of a cast, and change in this ratio may alter these properties.
What is the ratio for mixing plaster and water for this task? ›
Generally, you'll want to use approximately a 1:1 ratio of plaster to water—in other words, half and half. However, you should add only about half of the plaster at this point as more will be added later. Try not to take more than a couple minutes to sift the plaster, or it will begin to set.
Why is consistency and water to plaster ratio important? ›
The higher the consistency number, that is, the more water required, the softer and weaker the final plaster that results. This is because plaster becomes solid by the formation of tightly interlaced gypsum crystals; as more water is added, these crystals are pushed further apart, thus making for a weaker structure.
What happens if you add too much water to plaster? ›
The above table is based on USG® No. 1 Pottery Plaster mixed to a consistency of 70 (70 parts plaster to 100 parts water) recommended for most studio applications. Excessive water yields a more porous but more brittle mold, and less water means a very dense, hard mold that will not absorb water.
What is the best ratio for plaster? ›
Cement: Sand =1:5, for the first coat of plastering with 12 mm thickness. Cement: Sand=1:3, for the second coat of plastering with 8 mm thickness.
What is the mixture of plaster and water? ›
The general ratio for Plaster of Paris is two parts plaster to one part water. This ratio can vary. You will know to stop adding the plaster when the water no longer absorbs the plaster as it is added. Step 2Always add the plaster to the water (not the water to the plaster) and sprinkle in the plaster a bit at a time.
What is the water requirement for plaster? ›
To determine the water quantity, you simply need to take 25% of dry ingredients = 25% of 3207.96 kg = 801.99 kg = 801.99 litres. To sum up, To plaster a 10 m x 10 m wall with 15 mm plastering thickness and 1:4 plastering ratio, you would need 574.56 kgs of cement, 2633.4 kgs of sand and 801.99 litres of water.
Water-to-cement (w/c) mass ratio is important because it controls the mechanical properties and durability of hardened concrete. When problems occur and/or non-compliance with the specification is suspected, it is often desirable to be able to determine the w/c ratio.
What is the water-powder ratio? ›
Water/Powder Ratio: (As per IS 10262- 2019, C: 8.1) Powder refers to materials of particle size smaller than 0.125 mm. It includes this size fraction from cement, mineral admixtures and aggregate. The water/powder ratio shall be 0.85 to 1.10 by volume.
How does water powder ratio affect the strength of gypsum? ›
Type III and IV gypsum samples prepared with increased water-powder ratio showed statistically significant reduction in the Compressive strength and Brinell hardness values.
What is the ratio for mixing plaster? ›
Water to Plaster Mixing Chart
1 Pottery Plaster mixed to a consistency of 73 (73 parts water to 100 parts plaster) recommended for most studio applications. Excessive water yields a more porous but more brittle mold, and less water means a very dense, hard mold that will not absorb water.
What is the ratio of casting plaster to water? ›
On average ratios are 850g of plaster to 1 pint of water. Slowly pour plaster into the water and stop when the mound of plaster powder starts to show above the water. Mix gently, but quickly, until you have a smooth creamy consistency. Try not to whip/beat the mixture, as this will create air bubbles.
What to do if plaster is too watery? ›
If it's still too watery then simply add more plaster a bit at a time until it feels right. There is no 'perfect' in the consistency of plaster but in general, if you stick a small piece of wood in the mix it should be able to stand upright.
How thick is plaster mix? ›
Mix sand and cement until the colour is uniform. Then gradually add water while mixing until the right consistence is reached. If plaster is applied in a single coat, thickness should be 10 - 16 mm. A single coat should not be thicker than 16 mm.
What consistency should casting plaster be? ›
The correct consistency to aim for is that of single cream. Mix thoroughly making sure everything is mixed off the bottom of the bowl and then let it rest.
How thick should each coat of plaster be? ›
Undercoat plasters are usually applied at 11mm and most finish coat plasters are applied at 2mm.
How thin should plaster be? ›
Usually, thicker plaster is used on uneven walls and to cover large wrinkles. Thin plastering, on the other hand, is used to achieve a smoother finish and display the features of the wall beneath. Nonetheless, even with thin plastering, it should not be less than 6 mm to ensure its durability and strength.
A basic property of any gypsum plaster is its "normal consistency", or the amount of water required to mix a given amount of plaster to a state of fluidity. Both the gypsum plaster and the water are measured by weight, and the required amounts are expressed as a numerical ratio. A mixture of 1kg of water to 1.5kg of gypsum plaster, for example, can be expressed as 2 parts water to 3 parts plaster, or a 2:3 ratio. Usually, the equation is expressed in terms of 100 parts of plaster. The 2:3 ratio would then become 67:100. When 100 parts of plaster are added to 67 parts of water, the mixture has a particular consistency and this is expressed by the number 67 alone. The number 80 would express the consistency of the mix when 100 parts of plaster are added to 80 parts of water.