The Curling Manual

Table of Contents

Section 9 The Field of Play

The term "Field of Play" refers to ice and rock conditions. In this section, field of play is discussed as it relates to the competitive game.

Topics included in this section are:

  • Field of Play Standards
  • Rock Performance
  • Championship Ice
  • Assessing the Field of Play

Field of Play Standards

In some countries rigid field of play standards have been developed for championship play. To date, the World Curling Federation has not developed a set of standards for this area. In the United States, for example, championship field of play conditions must meet the following standards:

Standard Acceptable Deviation
Rock Type Trevor granite with blue hone inserts Well-matched rocks
Rock Pairings Closely matched by pair None
Ice Curl Four feet both ways 1/2 foot either way
Ice Speed 24 - 25 seconds ends 1-11 None
Ice Durability Pebble conformation ends 1-11 None

The above standards were developed by athletes and Board members. They represent a good balance between ice that plays well for the athletes and ice that is suitable for an aggressive game, where lots of rocks are in play. Television audiences prefer the action of a house cluttered with rocks. Ice-makers at US championships must meet this standard or risk not being accepted at future events.

Rock Performance

Championship rock performance and ice conditions go hand in hand. The performance of the rocks is generally related to the aggressiveness and consistency of the running surface. As stated earlier, differential pressure from a rotating rock creates uneven friction and frictional melting. The term "aggressive" refers to the running surface that creates good overall friction. The rougher the running surface, the more friction is created. Rock-makers create a balance between a rough surface that will curl and a surface that will still run at speeds within the standard. A more difficult task for the rock-maker is creating perfectly matched running surfaces. All rocks on a given sheet must perform the same.

Running surfaces (running edges) don't last forever. The manufacturer shapes the edge on the lathe with cutting tool. When the rock is placed into service at the cub, it begins a slow deterioration process. The edge can either wear away or "pit". Aggressive edges smooth out as they age and must be retouched occasionally. Some ice techs believe rocks should "worked" before every large event. Pitting occurs slowly as the rocks expand and contract. A pitted edge is very inconsistent in how it plays.

Types of Rocks

Blue Hone

Blue hone (from Ailsa Craig) granite is light gray in color and has a smooth, tight grain. These rocks have superior running surfaces and are usually well matched. Blue hone rocks chip easily and are usually chipped at the striking band due to their poor striking characteristics.

Trefors (pronounced Trevor)

Trefors are quarried in Wales and come in different colors, red, brown, gray and blue. The striking bands are usually in good condition due to the great striking characteristics. However, depending on the age and care of the rocks, the running surfaces are probably in poor condition due to pitting. These rocks must be checked carefully for a proper match.

Common Green

These rocks have a distinctive, green, speckled color. They are not particularly good rocks and should be checked carefully like the Trefors.

Inserted Rocks

A common practice today is to insert Trefors and common greens with blue hone running surfaces. This is done by coring-out the running surface of a rock and gluing in a blue hone insert. As far as matching these rocks, they will behave like a solid blue hone with better striking characteristics. These have the best of the best as far as overall playing characteristics.

Championship Ice

Creating a championship ice surface is very difficult. Twenty to thirty different variables come into play during the process. Before an ice technician can begin working with the variables he or she must understand the ice performance standard.

Basic Ice Maintenance

To achieve to above standard, the ice must be maintained using some basic maintenance techniques. Ice is scraped daily to remove the pebble from earlier draws. A proper scrape completely removes the old pebble so a new pebble can be applied. In many clubs, the ice is either not scraped daily or under-scraped. It may also be over-pebbled. This means the pebble will build up over time, creating and course surface which be slower and straighter. The ice will also slowly increase in thickness.

In championships conditions, the ice is scraped after each game. This completely removes the old pebble before the new pebble can be applied.

Ice Performance

Several factors come into play when trying to achieve the championship standard. We break them down into three categories:

  1. Facility related
  2. Ice tech related
  3. Athlete related

Facility Related

Ice sub floor - Sand based vs. concrete base and the amount of insulation under the base. Concrete is preferred due its durability, levelness and even distribution of cooling. Sand bases can cause pipe runs due the uneven cooling of the feed and return cooling pipes.

Overhead lighting - Light creates heat. Certain lighting systems create more radiant heat than others. Fluorescent lights create almost no radiant heat.

Ice paint - In some cases, dark painted ice can absorb heat more than lighter colors.

Radiant energy from the sun - Radiant heat from the sun can penetrate the roof and ceiling. A barrier can be created to reduce the radiant heat.

Compressor Cycling - The capacity of the compressor(s) will dictate how fast the heat can be "pulled" from the ice bed. Temperature control set points will determine the temperature range of the surface. Large capacity compressors combined with tight set points (a tenth of a degree) will allow the ice technician to maintain a constant surface temperature.

Heat and Humidity control - Heat will suspend moisture. Dehumidification will remove moisture. A combination of the two is preferred. Heat will also create a load that the compressor will have to pull out. The humidity in the rink should be controlled to provide a proper amount of humidity. High humidity will result in frost. A low humidity environment will result in "sublimation" which means the ice is changing from solid to vapor without passing through the liquid stage.

Ice Technician Based

Amount of pebble - The ice tech can control the amount of pebble applied. He or she will consider the type of play and length of game. Generally, after a clean scrape (scraping the ice back to flat with no pebble build-up) the ice tech will apply a double pebble with either a medium or small pebble head.

Size and shape of the pebble - The size and shape of pebble will depend on the temperature of the ice surface (freezing base) and the temperature of the pebble water. Warmer water is desirable because it melts into the surface and freezes in a rounder form. Cold pebble freezes on the surface with a tall shape.

Pebble water quality - Pebble water should always be a good quality de-ionized water. Pebble water will adhere best if there is no gas or impurities in the water. The ice tech can de-gas the pebble water by letting it stand open for several minutes after heating.

Pebble water temperature - As mentioned above, warmer water is best for pebble.

Heat - The ice tech can control the amount of heat in the rink area. Heat suspends moisture and will not allow it to condense on the ice.

Humidity - Humidity can also be controlled.

Athlete Based

Weight of the athletes - Heavier athletes create more friction on the pebble with their shoes. A men's draw will break down pebble faster than a women's draw.

Types of shoes - Hi-grip shoes will also cause more friction. Players wearing a slider when sweeping will create less friction.

Types of brooms - Synthetic brooms cause more friction than hair brooms. This is why they work better.

Ice Technician Control Points

A good ice technician will asses the facility and the rocks and develop; a plan of action. There are three main areas that an ice tech must control:

  1. Ice surface temperature
  2. Ice surface moisture
  3. Pebble durability

Moisture is an ice technician's biggest problem. It causes condensation on the surface which leads to frost. Moisture lubricates the running surface, causing straight ice.

Assessing the Field of Play

Good teams have a plan to assess the field of play before each game. Below is a sample plan for assessing the conditions.

Assessing the Ice

Assessing the field of play conditions is not the same as reading the ice. Good teams understand ice and the variables that affect performance. This section covers three ice variables a player should know.

  • Ice surface temperature
  • Pebble size and amount
  • Climate control (heat and humidity)
Surface Temperature

Teams should know the ice surface temperature at the beginning of the game. They should also know when the compressor "pulls". Surface temperatures in the 21-22 degree range will produce a slower, straighter condition. Temperatures in the 23-24 range will produce faster and swingier ice. Know the factors that affect the surface temperature during a game. There is no such thing as a constant surface temperature. At night, when the lights are off and the ice is not in use, the refrigeration system will maintain a surface temperature in the set range with very little margin of error. Large heat loads (lights, players. heaters, etc.) are difficult to remove quickly resulting in a spike of surface temperature. For example, a surface that will remain at or near 22 degrees when the ice is not in use may increase or spike to 25 degrees when the players start the game. The capacity of the plant and the set points dictate how high the spike will be.


A curling club maintains a 22 degree surface when not in use and the lights and heaters on. When all of the players enter the ice area and begin to play, an enormous heat load is added to the static condition. Each player represents a certain BTU (British Thermal Units) heat load. This heat will be transferred to the ice surface and the compressor will have to pull it out. There is a lag between the addition of the heat load and the compressor sensor. In some cases, the compressor is unable to pull the heat out fast enough and the surface temperature rises. When players leave the ice (removing a heat load) the surface will either remain constant or drop (if the compressor is pulling).

Begin your ice assessment by having a discussion with the ice technician. Prior to the competition, ask to see the refrigeration plant and ask what the compressor cycles on:

Sensing Area Short Result Long Result
Brine temp Long lag time Large temp range
Bed temp Medium lag time Med. Temp range
Surface temp Short/No lag time Small temp range

Ask the ice tech what the ice surface temperature will be at the beginning, middle and end of the game. Ice with surface temperatures in the low 20's may be straighter and slower. As the temperatures rise, the rocks curl more and it becomes faster. This is the case until about 27 degrees. Temperatures higher than that create soft, unplayable ice and there's probably something wrong with refrigeration system. The pebble will last longer in lower temperatures and will break down faster in higher temperatures.

The key to managing the surface temperature is knowing when it changes. Everyone has experienced a change in speed and curl after the all the other sheets have left the ice. With four to six sheets of people on the ice, the refrigeration system is working to remove the heat load (BTU's). If the heat load leaves quickly, the compressor lags behind and continues to operate. The surface temperature can drop a full degree by the time the system shuts off on temperature. In addition, the heat source offered by the players is no longer available, resulting in the air losing its moisture-suspension qualities.

Pebble Size and Amount

Ask the ice technician what size pebble he or she plans to use. A medium pebble will last longer while a fine pebble will break down earlier.

Heat and Humidity Control

Ask the ice tech if the club or arena has heat and/or dehumidification. Dry air has less moisture than humid air. With no moisture in the air (dehumidification) there will be no condensation on the surface. This means no visible moisture (frost). Early condensation looks like a dull film across the ice. You will notice that the sweeper's shoes will displace the moisture and cause a smooth spot. The moisture taken up by the shoes may even leave a spot on the ice where the shoe stops. Dry ice conditions promote speed and curl. Moist conditions lubricate the running surface and prevent curl.

Assessing the Rocks

At the higher levels, your ability to assess rocks is critical. Games have been decided on a team's inability to spot a bad rock and deal with it. The following is a guide to rocks and how to assess them.

There are many different types of rocks in play across the world. Wear and tear on these rocks differ with the type of granite used and the amount of playing time they get. As mentioned earlier, some granite is susceptible to wear and pitting causing them to behave differently. Mismatched rocks are a growing concern. With the good ice conditions we have today, particularly the speed, mismatched rocks can play an important role in the outcome of the game. As rocks age, the running surfaces change shape. They either wear (get flatter) or they "pit" which means small pieces of granite break lose due to constant freezing and thawing (expansion and contraction). This aging process is important to advanced curlers because it rarely happens evenly across a given set of rocks, causing the speed and curl of some rocks in the same set to differ. Most players use the rock numbers to throw rocks in sequence but there is no guarantee that these rocks travel over the ice at the same speed or in the same manner.

Know the Granite

Your first step in rock assessment is knowing what granite you're playing with. Most rocks today are Trefor granite with blue hone inserts. This is the best rock type. The running surface is made of blue hone which is a durable running surface. Visually inspect the running surface and determine if the rock is aggressive or conservative. An aggressive running surface will feel rough to the touch. The aggressive running surface will curl more with less speed.

Matching Rocks

Mismatched rocks (one slower or faster than the other) can greatly affect your ability to calibrate draw weight. A rock that is ten feet slower than the other will cause you to over compensate on each throw.

The best way to determine if two rocks are running at the same speed is to throw many, many draw shots on a good quality ice surface. The larger the sample size, the more accurate the data. Record the results and use the information in the future.

Steps for assessing rocks during a competition:

  1. Ask the locals for rock information
  2. Visually inspect the running surfaces
  3. Throw the rocks in practice and look for patterns
  4. Run the rocks together
  5. Track game patterns
  6. Watch other teams throw them

Throw the Rocks

In practice throw the rocks that you will be throwing in a game. Start by playing the rocks straight up the numbers. If you suspect a mismatch move to step 2.

Run the Rocks

If you suspect a bad rock, run the two rocks one in front of another for about fifteen to twenty feet. If the rocks are matched, they will run together in both directions. If one is faster, it will pull away from the other. This is not an exact science. Use this process only to determine really bad rocks.

Look for Patterns

Since most players do not have the luxury of running rocks (particularly at other clubs), the next best thing is to watch each rock closely for signs of unusual behavior. Designate a player to watch the rocks and the tracks on the ice. Bad rocks usually create a pattern (always light with a good split, for example). Always confirm split times with the team timer if you suspect a bad rock.

If time is not a concern (rocks at your own club) a good way to match rocks is to time them. If you have an opportunity to throw practice rocks, or you want to match rocks at your club, throw as many rocks to tee line as you can. Time them from hog to tee. This will give you the rock's speed in seconds. When complete, match the similar times. Teams using "split" or "interval" timing can match rock speed quickly by monitoring the splits. Consistent rocks will have consistent times in either case.