The term "Field of Play" refers to two essential parts of the game, the ice and the rocks. In this section, the field of play is discussed in depth.
Topics included in this section are:
A discussion of modern day curling and the success of your club, events, team, programs, etc. would not be complete without talking about the ice. Today's curlers expect the ice conditions to be fast and true. However, the ice is constantly changing and will change from club to club, event to event and even during the game. Local ice technicians study the control factors, listen to playability feedback and make adjustments to always improve the ice conditions.
In addition to your good delivery, sweeping mechanics and strategy, the ability to assess and monitor the ice (and rocks), will determine you team's success.
Ice playability has evolved rapidly in the last twenty years to thirty years. The invention of the curling ice scraper and the use purified water has changed the game. The ice of the past; slow, straight and dirty, is no longer acceptable.
Modern Ice Performance Standards
What do we expect when it comes to the ice? 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 printed standards. In the United States, for example, club and championship field of play conditions must meet the following standards:
The above standards were developed through a partnership between athletes and US governing body. 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.
See the section dedicated to Timing Rocks for a full explanation of how and why we time rocks.
To achieve the above standard, the ice must be maintained using some basic maintenance techniques. Ice should be scraped daily to remove the pebble from earlier games. A proper scrape completely removes the old pebble so 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 a course surface which be slower and straighter and keep dirt from being scraped away. The ice will also slowly increase in thickness as the pebble builds-up.
In championships conditions, the ice is leveled during the pre-event preparation and is scraped after each game. This completely removes the old pebble before the new pebble can be applied.
The information below is a guideline for ice making. Every facility is different and ice standards may be different. Work with your facility to make the best ice possible.
Club Ice Maintenance Recommendation
To re-pebble the ice during back-to-back games, adjust you're your time and coverage. A 30 second, 80 swing pebble could work. Be sure to nip again.
Several factors come into play when trying to achieve the ice playability standard. We break them down into three categories:
Ice sub floor
Radiant energy from the sun
Heat and Humidity control
Finding the right heat and moisture combination depends on your needs, budget and equipment capabilities. Remember, the more heat you add to your ice shed, the more heat needs to be pulled out with the compressor. Burning fossil fuels to heat a large ice shed will be extremely inefficient without a heat recovery system. A "closed system" is always preferred but not always practical. In a closed system, fossil fuels are burned to heat the shed. The warmer air in the shed reaches the ice and causes the compressor to pull it out. Then the heat and is dissipated through the condenser and back into the atmosphere. The closed part of the system is a heat recovery process that takes the heat from the condenser and uses it again, back into the ice shed. It's not 100% efficient but a lot better than losing the heat to the atmosphere.
Amount of pebble
Size and shape of the pebble
Pebble water quality
Pebble water temperature
Ice Surface temperature
Humidity and Dew Point
Rock running surface
Weight of the athletes
Types of shoes
WCF Tournament Compliant Broom Heads
Ice Technician Control Points
A good ice technician will assess the facility and the rocks to develop a plan of action. There are four main areas that an ice tech must control:
Moisture is an ice technician's biggest problem. It causes condensation on the surface which leads to frost. Moisture lubricates the running surface, causing straighter ice.
As stated in earlier sections, curling rocks are made of solid granite quarried in only two places in Europe. Rocks come in different types, mainly distinguished by the types or types of granite used.
Trefor (pronounced Trevor)
With the three main granite types, rock types are either solid trefor, blue hone or common green or a combination of the granite types.
Let's understand what is happening underneath the rock as it travels down the ice. Curling rocks are approximately 12 inches in diameter; however, there is a smaller, ringed portion that the rock rides on. This narrow ring is about 5 inches in diameter and is called the running surface.
Rocks are intentionally rotated either clockwise or counterclockwise when thrown. Intentional rotation provides the necessary degree of predictability as the rock travels down the ice. Most rocks, if thrown without a rotation, will assume a rotation at some unpredictable point. As the rock is rotating, one side of the running surface will always be moving faster than the other as it travels over the ice surface.
Example: If a rock traveling down the ice has a clockwise rotation, the left side of the rock is traveling faster over the ice than the right side.
For most of curling history, no one was clear about why rocks curl. A breakthrough 1990's study by a University of Northern British Columbia physics professor, Mark Shegelski identified the physics under the rock (described below). In 2015, a Swedish study by Harald Nyborg of Uppsala University in Sweden provided a different insight into the physics. This brings up the two main theories of why rocks curl.
Why Rocks Curl Theory #1: Differential Frictional Melting
How Sweeping Affects Differential Friction
Now add sweeping. The sweeping motion briefly polishes the ice (pebble) just before the rock travels over it. The sweeping action melts a molecular layer of ice for a very brief moment, resulting in a molecular layer of moisture. This creates an even lower friction environment that helps the naturally occurring frictional melting. This combination allows the rock to decelerate slower. This results in the rock traveling farther. The overall reduction in friction has another effect: Since the rock is dragging less on both sides, the rock will travel straighter.
Sweeping cannot make a rock move faster, only farther!
Why Rocks Curl Theory #2 - Grooving/Scratching
Which is Correct?
See the Section on Sweeping for more.
Rock Performance Standards
Rock Performance & Maintenance
Treat your rocks with care. Limit their use by NOT using them to drag the ice before games. Rotate the rocks by sheet occasionally since most practice rocks are thrown on center sheets.
The rock's running surfaces get dirty by picking up small dirt on the ice. Small pieces of rubber from deteriorating grippers can attach themselves to the running edges. Solvent clean rocks running surface once per week.
There is a method to gain aggressiveness in your rocks. Scratching or texturing the rocks will roughen the running surface and gain aggressiveness. This method is very popular in the competitive world as the world class curlers demand more and more rock performance. It is not uncommon to scratch the rock mid-way through a world or provincial championship. This method does not actually sharpen the running surface. It roughens it.
Rock manufacturers will agree that texturing the rocks should be the last step in the overall ice/rock performance plan. There are several ice maintenance tricks that will give you more curl without touching the rock's running surface.
Every time a rock is played, practiced with, and/or used for dragging it is slightly polished by the ice friction. The more the rock is used, the more polished it gets. This gradually changes the rock ice interface.
If you decide to texture your rocks, use a texturing jig to roughen the running surface. The jig allows you to consistently sandpaper the running surface. Several methods can be used based on how aggressive you need them.
On the other hand, warmer, freshly scraped ice has less friction. Nipping instead of running rocks will lengthen the interval.
*Note: Contact rock manufacturer for more information and any possible warranty issues.
Determine how many rocks throws occur at your club.
Determine if your rocks are high, medium or low use by counting the number of times each rock is thrown per week. Count your regularly scheduled draws, practice sessions and the number of times the rocks are used to drag. Multiply that number by the number of weeks in your season.
If you keep your ice warm (23-25 degrees) which is recommended texture your rocks every 2000 throws.
If your ice is cold and/or frosty (20-22 degrees) texture every 1500 - 1800 throws.
Using the texture jig and aluminum oxide paper. One piece per rock.
Field of Play SummaryChampionship 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. It also creates deeper grooves in the ice. 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 a cutting tool. When the rock is placed into service at the cub, it begins a slow polishing and deterioration process. The edge can either wear away or "pit". Pitting occurs slowly as the rocks expand and contract. A pitted edge is very inconsistent in how it plays.
Aggressive edges smooth-out as they age and must be retouched occasionally. Some ice techs believe rocks should be "worked" before every large event. "Working" the rocks or "scratching" the rocks is achieved by sanding the running edges with sandpaper or emery. Different techniques exist but they all involve turning or dragging the rock on a piece of sandpaper. This roughens the edge and makes the rock more aggressive. It also helps gain consistency in the rock sets.
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
The type and frequency of the ice scraping will have huge impact on the ice speed and curl. Understand how often the ice is scraped and if the pebble is completely scraped off before it is pebbled and nipped again.
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 23 degrees when the ice is not in use may increase or spike to 25 or 26 degrees when the players start the game. The capacity of the plant and the set points dictate how high the spike will be.
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;
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.
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.
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.
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
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:
Throw the Rocks
Run the Rocks
Look for Patterns
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.
At first glance, the sheet of ice, excluding the houses, has many parallel lines and other lines that intersect at 90-degree angles. In particular, the center line and the four-foot lines, if in use, represent reference lines for throwers and sweepers. However, only a few shots in a game will run parallel to these lines. Since all shots are thrown from the hacks that are fixed a few inches away from the center line, most shots will initially travel at an angle to the center line. Compounding the issue is the draw back point of the individual player. Some will draw back on the center line while others will draw back to the hack toe. This represents four inches or so at the front end of the line of delivery (imaginary line between the skip's broom and the starting point).
The Outturn Issue
Arena Ice Considerations
With the popularity of curling growing every year, new clubs are forming without dedicated curling facilities. By acquiring rocks and basic equipment, new clubs can form using ice at the local skating rink. This is a great way to enjoy curling at a low cost. Curling on arena ice does present some challenges. Most arenas share ice with hockey and figure skating. Preparing the ice temperature and surface for each group looks different. Hockey players want a cold, flat surface at about 20 degrees. Figure skaters want a warmer, flat surface at about 25 degrees. Both hockey and figure skating don't necessarily need a level surface. Curlers want a level, pebbled surface at about 23 - 25 degrees.
All preparations for daily use involve the Zamboni. The Zamboni is an efficient machine for prepping hockey and skating ice. For curling however, it can be a problem. The machine scrapes the surface and lays water down to fill holes and create a seemingly flat surface. It does not create a level surface. Even the curling scraper can't level the surface properly.
A surface that's not level may produce a "negative ice" situation. Negative ice happens when the surface is not level. The physics under the rock that make it curl no longer exist (see Why Rocks Curl). In this case gravity may make the rock curl, even against the turn. Our staff has arena experience and they report situations where rock curl twelve feet with the turn and back up eight feet against the turn.
Your game strategy and shot calling may have to be adjusted to make shots on the non-level surface.
Tips for Playing on Arena Ice
Throw more draws than takeouts. Even on a non-level surface, draw weights can be normal. A team that can fill the house with draw shots has an advantage since the takeout is less predictable. We've seen teams throws eight takeouts in a row and miss everyone.
Is the ice is running negative, gravity is making the ice curl and not differential frictional melting. If gravity is making the rock curl, sweeping makes it curl more. Of course this is the opposite result from normal curling.
Measuring Equipment and Procedures
Occasionally, two or more rocks are too close to measure by eye, whether it is to the tee or around the perimeter. Measurements come in two categories:
Measuring the rocks by device will help. There are four types of measuring devices available at most clubs.
The first and most often used device is simply called the "measure". It is used to determine the counting rock or rocks in the house. There two types of mechanical measures, the spring gauge and the manual touch no touch device. Since the spring gauge is a mechanical device with moving parts (springs, levers, etc.), there may be some inaccuracy on very close measures. The TNT device eliminates the mechanism and the determination is made based on where the device touches or not. This is far more accurate. The spring gauge device can also be used as a TNT.
A note on both devices:
The Laser is an electronic device that reads distance via laser and provides a reading in 100's.
The six-foot is used to determine whether or not a rock is in the house. It is also used as a perimeter measure at the back of the house at the intersection of the center and backlines. Unlike the above device, it may be used during the end before all eight rocks have come to rest. There are only two reasons to use the six-foot measure during an end:
The third measuring device is called the "90 degree" measure. It is an L shaped piece of metal use to determine if a rock is in play around the perimeter of the playing area.
Measurements are needed when the teams can't agree by eye. A quick note about measuring rocks. The object of the game, of course, is to get closest to the tee, which is an exact spot where the center line intersects the tee line. Measurement by device actually measure s closest to the center of the hole that is drilled over the tee. In some cases, where the hole is drilled improperly, erroneous results could occur. In addition, the standard measure has a cone shaped tip that fits into any size hole. The Laser measure may have a smaller tip and may not fit slush into the hole. Always push the laser toward the rocks being measured.
If officials are not present, the vice skips are responsible for measuring rocks if necessary. The following is the correct procedure for measuring. Rocks are measured clockwise from the back of the house. This is the standard.
Too Close to Call?
Measuring Two Rocks (standard championship procedure)
Measuring Three Rocks
In both situations it is acceptable to swing the device back to the first rock for a closer look. If rocks cannot be determined by device, a blank end will result. This is very rare. If the measure is very close, consider moving the mechanical sensor "away" from the rocks. This reduces friction between the sensor arm and the rough rock running surface. If the sensor arm cannot provide a reading, visually determine.
If two or more rocks are so close to the button that the device cannot be used, a decision must be made visually. Find an impartial person to do this for you.
Using the Laser Measure
Using the Six-Foot Measure
If, during a free guard zone measure, another rock is in the six-foot path, a decision must be made visually.
Using the T-Square
Visually check the bottom of the device compared to the line.
In championship play, where the Last Stone Draw (LSD) is used to determine hammer in most games, rocks are measured from the center of the rock. A tape measure or laser is used for the initial reading from the closest edge of the rock. Then, the radius of the rock is added to complete the distance measure. It may be necessary to measure two rocks that both cover the tee. At the highest level, it is common to cover the tee on more than one shot. The official in charge would take measurements from two spots, one on the tee line at the intersection of the four foot circle and the other on the center line at the intersection of the four foot. With these two numbers a mathematical formula can determine which rock's distance from the tee.